Coming from the other side of this... I grew up in a very passive aggressive household and experienced similar confusion towards my mother and sisters while age 10-18. I am ashamed to admit I stared a lot and groped my sister on one occasion but guised it as "playing." Nobody ever said a word. But I wish they had.

I believe the issue stemmed from my mother's passiveness and shaming. She set high standards for how I was to treat women, but never communicated healthy ways to handle attraction and never called out my behavior. My mother had been molested by her brother once and I believe she projected this fear while also ignoring any signs I gave of exhibiting similar behavior so she could avoid the pain of the memory.

There is a lot of good advice in this thread. Be sensitive of shaming him, but please communicate these things to him. I have yet to form a meaningful relationship and wish I'd dealt with these issues at a much younger age.

From my psyche; you seem to have more reasons to believe that he is attracted to you beyond him touching your crotch. But it is worth considering he might simply curious about the size of your penis and how he compares. It does not make the behavior okay, but he might be coming from a different perspective than you think.

What he needs most is open, honest communication that affirms you love him and want to help him be a healthy, whole human being.