N.B.
Absolutely normal. Just teach him that it's a private thing and that you don't want to watch or see him doing it. Instruct him that, since it's private, it can only happen in his bedroom with the door closed and he's alone or the bathroom.
Boys do this and they get erections when they watch TV ads and TV shows and more. Learning it's okay and where to touch it is a HUGE plus in anyone's books.