Every year when it gets close to my birthday, my son starts sulking and saying it's not fair that it's going to be my birthday and not his. (His birthday is 2 weeks after mine.) When it comes to the actual day itself, he makes it a point to get upset when anyone gives me attention or says happy birthday to me. It makes it a real trial to be around him on my birthday because he always does everything to bring the mood down. I just don't understand why he's so upset and what I can do to help him cope so he stops taking it personally when anyone else gets attention. He's an only child and tends to get upset for the smallest reasons anyway, but I can normally find some rationale for the behavior. But this continues to baffle me every year. Does anyone have any advice?
For only children, in my experience, it's pretty common for everything to be "about them" most of the time. So when it's not, it's surprising to them! This is particularly common with only children with a stay-at-home mom in my experience - they have a person who's around them all the time, and who's taking care of all of their needs.
I think that empathy is the first place to look here. Talk to him about how you're feeling in terms of how he feels on HIS birthday. This probably isn't the only place that needs this kind of work, though - do some reading on how to help children develop empathy in general, and in particular see if you can find out specifics on only children, as they really do have challenges that are different from siblings.
I would also address the specific fits fairly directly. Make clear how you're feeling as a result of his actions, not in a mean or stern way but just matter-of-fact. If he's being mean, put some distance in time and space - let him know you're feeling sad because he's being mean to you, and you're going to go somewhere else for a few minutes so you can feel better. Be consistent her, and don't imply it's a punishment - it's just you taking care of yourself. This will help him think about your feelings, and understand them, and put names to them.
Then, at a time when he's not throwing a fit, talk to him about how you feel about your birthday, and help him connect that to his feelings about his birthday. Understand his feelings - and don't say they're wrong. Instead, help him understand your feelings. His feelings will have to change, but they're not going to change just because you say so - he has to think about it differently, and that has to come from him.
Ultimately, I think that it's important to understand on your side that his dislike of your birthday is probably primarily that it takes the focus off of him, where he's normally the center of attention. It's not that he doesn't like your birthday, it's that he sees the sum total of attention as zero-sum, and wants all of it. This will take some work to overcome; it may help a lot when he goes to school, if he does that in the fall, but it is something I have seen in several only children that I have known. They're used to being the center of the world, and when they're not, they don't understand why not.
Why is he upset?
Young children often don't see things which are fundamental truths for adults as inevitable. To you and me it is obvious that every person has exactly one birthday per year, and that there is no interaction between those birthdays. Your son might instead view birthdays as a communal resource, and every birthday claimed by someone else is one less for him. Or he can simply conceive of the alternate reality where he gets many more birthdays, and he then feels frustrated about it.
Kindergardeners are still in the process of developing a full "theory of mind" . This means they haven't fully realized yet that other people are indeed people, too. Thus, they will often need some assistance to accomplish stuff like being happy for others. The difference between "this is unfair" and "I don't like the outcome" will not be that apparent to them either.
What to do about it?
This is a great learning opportunity regarding taking pleasure in other's happiness. As per swbarnes2's suggestion, have someone else lead him in taking an active role in the birthday celebrations for you. This person should explain how they enjoy contributing to others having a great birthday party. For third persons' birthdays, model the behaviour yourself. Besides being involved in planning, getting/creating a gift for you should also be on the agenda.
To make this all a bit more palatable, you could consider getting him something, too. This needs to be something minor, and not to overshadow the lessons in the preceeding paragraph. Some sweets perhaps, to soothe over any remaining pangs of jealousy.
 A type of experiment quantifying this is the Sally-Anne-Test. See https://en.wikipedia.org/wiki/Sally%E2%80%93Anne_test for an explanation and links to scientific papers.