It could be good natured ribbing.
But, if the OP’s son has some limitations in his development, it’s also possible that he’s an object of ridicule by people who are cruel and mistreat him, and in a fervent desire to be part of a group the victim misinterprets this as “friendship”, since they let him tag along with them.
I’d think it’s important first to figure out if your kid is being mistreated (I.e. Do they always dare him to do outrageous things? Do they routinely “borrow” his things?).
If not, then I’d broach the subject of how some guys will tease their friends, and how to roll with it, while still making it clear that a person must always remain true to their values, no matter how much peer pressure there is.
But if the kid is being taken advantage of, then I think the solution is to help him get away from the group.