Sons Sexual discovery....HELP!!

Hi there, the is my first time writing here and i'm hoping someone might be able to help advise me. My 14 year old son is Autistic, he doesn't speak and is still in nappies, but 2 days ago he suddenly wondered into the living room nake so i took him back to his room and put a new nappie on him, he did this 3 times but the 3rd one was dry so put it back on him and left the room, it seemed a little to quiet so went in to see if he was ok and to my shock the nappy was off and he was playing with himself, well i have spent the last 2 days fighting to keep his nappy on but today he kept it on but started pulling his "bits" out to the side of it and still playing with it, what should i do? i have had to warn friends and family to stay away out of worry for how it would make them feel if he did it infront of them. i am preying for a mirical i just want to cry as i was so not ready to deal with any of this yet.