My wife breast-fed one of our girls quite longer than 2.5 years (can't recall if it was 3.5 or even 4...); another one more or less weaned herself earlier. Both turned out fine.
In my opinion, there are two aspects for you as a mother to keep in mind:
- I encourage you to do as you please, and ignore "customs" as far as you can. It's not the public opinion that should decide for you. Go with your feelings.
- You are allowed and encouraged to make up your own mind, i.e. you also do not necessarily let your child dictate your actions. You and your son are a team, not a hierarchy, at this point, and you have as much say as he. We are used to do what babies want so they stop crying, but at 2.5 he should be old enough to pick up some other habits.
On 1.: obviously, if you get penalized for still feeding your "old" child, then at some point the penalizing (whatever it may be) may be more than you can stomach. in this case continue with "2.".
On 2.: penalizing is not the only problem. For example, if your skin gets sore from the feeding, or if you are just not feeling that it is "right", then you can let your child know that.
Obviously it depends on the age of the child, i.e. at 2.5 you can hardly sit down and talk it through. But you can offer alternatives, and maybe bring up a slight barrier for breast-feeding. I.e., try to make the pauses inbetween slightly longer, try to gently nudge your child away from the breast slightly sooner and such. But basically just flood the child with alternative sources of food, and with alternative sources of "warm, cosy feelings". Get in a habit of cuddling a lot with no breasts involved, and such.
But in the end, rest assured that eventually he will stop breast-feeding.
The old joke "Oh, you're still breast-feeding him, how old is he now? -- Oh, the wee one is only in his 72nd month..." helps, I guess. :-)