Dog biting his own penis.

I have 2 foster brothers who had been severely abused when I took them in. (Broken ribs, mouths tied shut, mites, worms... The list goes on) my big guy Jack gets into these stages where he bites & motions his penis with his mouth. Sometimes he even falls asleep with his mouth on it. His brother never does this. I have always had female dogs. Maybe this is normal behavior? Tonight like many other nights, I found him in the spare bedroom doing this. I don't scold him when I find hi doing this, but try & get him into another activity (take him Outside & chase a ball etc...) as I said, his brother never does this. Jack is 146 lbs and my vet & trainer think someone tried to do some kind of special breeding....his brother (same litter) is 65 lbs & looks like a pitty mix. After much testing experts have told me they do not have the same father. Anyway, is this okay? My trainer & vet say don't worry about it. They were both neutered at 5 months old. He also tries to urinate on my chow girl (spayed 12 years old) often. He is extremely high strung (observed by his trainer) he goes crazy if he sees a shadow or light move on the wall. Sorry for the babbling question. I don't ever scold him for doing this, but if it's normal should I just let him be during his time? Thanks.