[Jess~]

I tried googling for help on this issue and actually came across a few old TH threads with the same problems. in one of those thread's, he went to the doctor and it was a hormone overproduction problem.
since that is most likely the case here, as I don't know what else could cause this, it is vitally important that you get him to a doctor whether his mom approves of it or not. once he gets the problem fixed, his penis will stop growing, but I don't think it will ever shrink, so the sooner this is resolved the better. if a penis gets too big, it can actually become a painful disability, and I read of one case where a man couldn't have children or have sex ever again.

does he have a relative who could take him? or a friend's parent? look if there are any walk in clinics near you, and see if an adult could take him.
also, have him talk to his mom again, or you could talk to her, and tell her how serious an issue this could be an how it's worrying the both of you.

best of luck