I am 21, have been stuttering my whole life. He just has to adapt/deal with it (I’m sure he has at this point). I don’t know how much he cares or is bothered by his stutter, but when someone, my father for instance, tells me that nobody minds that I stutter, in the end, I still do mind so this remark holds no value to me. I had a more severe stutter when I was little, it got significantly better over time but it’s still there and will probably never go away entirely. But if you are bothered by the fact that he might not have that many friends due to his stutter, which I doubt, given his age cathegory, then the friends he would have made otherwise would have been fake to begin with. If he is engaging and not shy like myself, then he should have no problem finding new friends. But let me assure you, you can do absolutelly nothing about this, so just relax a let him deal with it.