It can in fact be related to autism and there are plenty of peer-reviewed articles on the internet about this. Here is one http://link.springer.com/article/10.1007/s00431-008-0766-2
I encourage you to read widely on this topic and not rely on opinions. Also seek the advice of a professional. look at evidence-based scientific articles. People may mean well in offering their opinion but it is just as easy for you to look up a scientific answer as posting a question on a discussion forum. Why not go straight to the source. Good luck.
I taught 4 year olds and had a problem with this in my room! I had a little boy who, constantly, was doing this! I did not scold or punish him OR use negative words when redirecting him. Don't make the genitals a negative thing for a child or else it will mess up their idea of genitals in adulthood. I like 'privates in private' idea pretty good! When I taught this specific child I am speaking of, I would just distract him from what he was doing the best way I could. Emergent curriculum worked best for this child- seems he only 'played with himself' when he was bored so when I would catch him doing it I would send him to another center or let him do fun art and painting- anything to catch his attention!
I agree, it's not just autism.
Keep it simple, say 'privates in private', and take him to his room/bathroom.
You can also start keeping a diaper/underwear on him (at least start trying to keep one on longer... I know a lot of kids like to be naked :) ).
Touching the genitals do not have anything to do with autism, at that age all kids do. You can not suppress something that is natural in any human being show that this is not done in public and when you want it to be in his room in private. If you go to your room and you see what is making it ignore it and leave the room, the most important thing is to teach that there is a place for that.
I can't be positive, but I don't think it has anything to do with his autism. My son did the same thing when he was 3 or 4 and he's not autistic. It may be harder to redirect your stepson though. Everytime I saw my son touching himself or stimulating himself, I would tell "you can do that all you want in private" and I would take him to the bathroom and close the door. After a few 30 minute sessions, he got tired of it. LOL.