This is complicated. I'll be blunt. I have a friend that went through this with his NT child. Since he is 14 his brain lovemap is almost set. It his highly likely he has a diaper fetish. If he doesn't have access to diapers he probably fantasizes about wearing them and thus the pictures. If you shame him he will not feel open to talk about his real desires. I think it's ok to ignore it and tell him to keep it private unless you want to be involved in his sex life. That's what my friend did. Don't ask, don't tell. But don't be surprised if you find diapers in his room like he did. A counselor would be good if he is receptive to it. If he can't help it there may be some OCD involved.

However, to mitigate this behavior and other self stimulating behavior he needs to participate in hobbies and have an active life. IMO, if you want him to stop say its no big deal in private, then give him healthy activities to wear him out. Also be frank with him about sex. He could be really confused. Now I come from a Christian background and believe fantasizing about women clothed or not is lust and takes away from the thrill of his future wife. I also believe that mast is healthy and if done when the body wants it than it can keep us from having impure thoughts. The two can be separated. The psychologist will probably lean towards acceptance and self control. The literature on it is sparse with almost no success once puberty hits.

I'm not an expert, just a parent. There are several questions in the Q+A section that address "abnormal" or unwanted sexual behavior in autistic children such as habitually touching and playing with themselves in public. I would research some of those solutions, and see if you could apply them in the case of your child. Can I ask you why it seems to trouble you so much that your son is behaving in this way?