How old is your son? At least he has the concept of doing that stuff behind closed doors. My ASD daughter is 13 and has a habit of sticking her hand between her legs. I don't think she does it in school or in public. She likes to get my attention by saying "porn hub". We were in a restaurant and she said "I feel pregnant." She had a stomach ache my wife didn't hear that remark. You or somebody else should talk to your son about sexual behavior dos and don'ts.