Stimming without realizing it is normal; sexualized contact is not. Perhaps this is a version of stimming, but when it came to things my son was doing that could have potentially gotten him in trouble with the law someday, I never pulled any punches. I would tell your step son that at his age, were he to do this in public (or in front of younger children, etc), he could find himself in jail, kicked out of school or fired from a job. Therefore, he has a vested interested in solving the issue. He needs to work with the adults in his life to find a solution so that he doesn't forget. With my son I found that quiet signals like touching him on the shoulder were effective ways to let him know he was doing something he should not be doing. Most of time these issues were reflexive actions he didn't fully realize he was engaged in. We didn't need to make an issue of it; we just needed to make him think about what he was doing. That may easily be the case for your step son; that he barely realizes he is doing it. He won't become aware and attentive to the prevention of it unless he is given a consistent reminder.
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).