Reddit mentions of When Young People with Intellectual Disabilities and Autism Hit Puberty: A Parents’ Q&A Guide to Health, Sexuality and Relationships

I found this book to be really helpful When Young People with Intellectual Disabilities and Autism Hit Puberty: A Parents’ Q&A Guide to Health, Sexuality and Relationships for parents and teachers.

If your son has good use of language, the book Things Tom Likes is about precisely this issue. It's fairly graphic in showing exactly what it is that Tom likes to do, but it's not tittilating or eroticized. It's meant for a target audience that needs very concrete information.

And this site has a lot of resources.

Sometimes the solution is to get a shoebox full of "tools" (Tissues, lubricant, pictures that he finds sexy - not necessarily porn at this age, maybe just models) + some step-by-step instructions reminding him to clean up and wash his hands afterward and keep that in his room. This shows him that you're not shaming him, you're supporting him if he wants to do this, but you're putting some boundaries on it (at home, door shut, etc.). Then if he does it in public, he can be reminded to go to his room and, if necessary, given a very mild punishment, but when he's in his room, it's obviously being allowed and supported.

Lastly, while you're sorting this issue out, you might want to consider what he's wearing. Some clothes rub against the skin in an arousing way. Some clothes allow easy access to the genitals. There's nothing wrong with having him wear clothes that make masturbation less likely when he's at school (as long as he can still toilet, etc.) and having him change back into comfy sweatpants at home.