Thankfully my son has a few other only children in his class.
He has asked why he doesn't every now and then and I say various reasons why but then come up with stuff like 'if you had a brother or sister you wouldn't have so many toys/fun things to do' etc.. and he is fairly happy with that.
Thank fully he is pretty easy going so moves on easily.
I have no helpful ideas, though, sorry.
Oh, does he have a pet, like a dog or cat to play with? we only have budgies but my son loves them and interacts with them a lot. lol