I have some...interesting experiences around this. I'm a known sperm donor and have donated to children that don't know I am their biological father, though none are 11 yet and so far they either were told something about the relationship from the start or haven't yet had 'the talk' yet. Still, I have also advised/heard from others who have had similar talks with children and have some experience handling the other aspect, being the biological father to a child you want to know while still needing to respect the parents and not confuse the child.
I second everything Willow said, so I mostly want to add to what is already said. Some of this is hard since some of this advice varies significantly based off of factors I don't know in your relationship; I'm trying to cover all the possible major points, so forgive me if some isn't applicable to your situation.
First, I don't know how much of the lack of contact was your choice or how you feel about it now, but let me stress even if you are upset, resentful, or otherwise unhappy about it, you should not express this to the child directly. Also try not to let the, understandable, feelings taint your relationship with your uncle or the mother, because ultimately anger/fighting/hostility between caregivers always filters down to, and harms, the child. It is perfectly okay to express and discuss feelings, even negative ones, with the parents so long as you don't let it prevent you from respecting their rights as parents to the child.
It's important to make it clear that he has two parents already and that won't change. That is to say that you are his biological father, but that does not change the fact that he has an adoptive father that raised him and has at least as much right to call himself the father as you do. You do not want to put him in a position where he feels he is expected to choose between you and your uncle, or that he has to doubt his relationship with his family.
You also don't want to create an environment where your uncle feels you are trying to take his place, usurp his authority, or otherwise affect his relationship with the child or that he is competing for the child's love. The child will ultimately suffer from any tension in your relationship with the parents, and could be placed in a position where he feels he's expected to pick sides. Keeping it clear that your biological relationship doesn't change his relationship with his parents and that you are simply another adult/parent that cares about him helps to avoid a lot of this confusion.
In advising people using known donors, one thing I've seen a number of times is the non-biological father being very sensitive about his role as father of the child, but either being unwilling/afraid to express his feelings or simply not wanting to cause trouble, leading to his pretending to not mind things that are secretly upsetting him. This then leads to the biological father, mother, or others saying or doing things that further upset the non-bio father, because they didn't realize it bothered him, until one day the non-bio father 'suddenly' has an apparent huge shift in his stance about things such as contact with the child because he was pushed too far past what he could ignore by people unaware there was a problem. In your case that's probably less likely then with known donor situations I'm use to, but I still advise you pay attention to your uncle's feelings as well as the child's and be careful about saying anything that could be seen as questioning his being a father to the child until/unless you're 100% certain he won't have a problem with it, as well as assuring him that you are happy to listen to and respect his feelings through this if he wants to express them.
How children respond to this news varies greatly, so I can't give exact advice other then to listen to him, answer his questions honestly and generally respond to what he wants. You shouldn't offer him counseling, or even act as if you think he should need it, unless he expresses sufficient confusion/difficulty to make it seem like it would help. Basically don't act like this is a shameful secret he should not be able to handle because then he is more likely to see it as that. At the same time listen to what he expresses and be willing to respond to whatever emotions he expresses.
One thing you need to prepare for is the question for why you weren't involved in his life and/or why he wasn't told he was adopted by your uncle sooner. This is a very difficult question because of the need to be honest with him while also making him feel cared for (not just discarded by you) and also not alienating the parents or, intentionally or unintentionally, placing blame on them for your not being involved sooner.
I suggest you speak with your uncle and his mother beforehand to decide exactly how to respond to this question. In particular to make sure that all three are in agreement with the answer and you don't upset them or make them feel you're blaming them. Stressing that he was happy in his life and you didn't want to make things confusing or worse for him when he was young helps, maybe even with a slight "you're old/mature enough to understand" now vibe. Letting him know that you were watching him and asking and basically staying informed about his life could help a lot here, to let him know you cared even if he didn't know your full relationship. If you can honestly say it being able to stress that the three of you were in agreement about when the right time to tell him could help; but I wouldn't lie to him about that fact if it's not actually true.
If you wanted more contact and do feel that your uncle or the mother prevented you from having it that will be harder to handle, since the need for honesty competes with the need to not overly criticize/alienate the parents. I would suggest something along the line of "your parents felt this was best and I respected their wishes". Basically being honest that it was their decision and not yours, but also showing that you understood it was their decision to make and you did/will support their decisions as parents anyways.
Because you are already family he is going to know that his extended family were all aware of this news and didn't say anything which is going to be another problem. It would be best to warn any family he is close to (like his grandparents or any other uncles/aunts) that you are having this discussion so they are ready when he asks them about if they knew. If you and your uncle can agree on what to tell the child letting the rest of the family know the 'script' on how to answer his questions about why no one told him sooner could help. Not all children will ask these questions, but best to prepare everyone in case he does.
Finally, you must not place expectations on the child after he knows the truth. The child may be interested in learning everything and want to come visit you all the time, or he may be completely indifferent to the genetic relationship and see you nothing more than 'just' the cousin he always knew you as, or he may outright be angry at you. Most likely he will mostly be confused at first and not know what he feels. However, whatever he feels you must respect it. That means not making him feel as if he 'should' feel something different. That also means not trying to force contact with him if he is resistant to it, while also being ready to have more contact and answer questions for him if he wants it.
Most likely he will be confused as to what he wants. He likely will be somewhat interested in more involvement with you while also completely uncertain how to go about it and feeling very strange/awkward around you at first. If that's the case suggesting some controlled intermittent 'icebreaker' forms of contact, going out to do something with him and his parents, could be a good idea in moderation, but be ready to respect him if he is not interested or just not ready for something like that.
On the other hand either you or his parents may not want him to have more contact with you than he already has. if that's the case you need to be ready for what to do if he seems more interested in contact than you're okay with. Expressing that you don't want to mess up his current family dynamic is a good place to start with that. Something along the lines of "they're doing such a great job taking care of you that I think it's best that things stay this way".
If he has a phone AND his parents have approved it giving him your phone number so he can contact you if he ever wants to or has questions can be a good step in either case. In my experience it's actually unlikely he will ever use it, or at least not unless/until quite a bit of time has passed for him to cope with the news and a relationship has formed between the two of you from some other means like in person visits. However, the act of giving him a means to reach you whenever he wants makes you more available to him and shows him you're interested in him. Essentially it's a great gesture that could make him feel better, even if it's most likely not going to be much more than a gesture without more complex relationship building.
If you still live 11 hours away from him setting up an optional time he can Skype with you could be something worth considering, allowing some controlled limited contact at a distance, but again discuss and get approval to do this with his parents before suggesting it and then wait to judge how he is feeling before you try to arrange anything.
