It sounds like overstimulation and ignoring personal boundaries on the part of your husband. I don't think there is a sexual component from what your wrote but it is definitely inappropriate.
Have you said "stop he is not enjoying that"? Or told your son it is okay to say "No daddy I don't like that?"
Poor little guy.
That's how it appears to me as well. I have told him in the past to say "please stop" when H was grabbing his toys away from him. It made H very angry and I risked him threatening D again. S3 is able to say "don't like" sometimes. Sometimes H stops when he says this, sometimes not. H has been paying attention to when I am looking now. I see him look to see if I'm watching and back off, but when I am not in the same room I hear screaching sometimes or grunts of frustration and it is very hard. S3 is still so young and wants the approval of H. It is hard to know what behavior he does not mind and what he is just tolerating for H's approval.