I would not call it boys being boys with that incident you mentioned between the cousins. Especially at his age. Even DS at 4 - if we saw that behavior with him and his friends, we would be concerned and all his male friends are 5 years old but none of his friends would intentionally do that to another of their friends. That sounds like bullying to me, when a child is trying to dominate another child like that (and if the other boy's dad noticed that B6 was acting oddly, chances are that B6 knows that was wrong too).
However, whether to bring it up to the parents or not really depends on your relationship to those other parents and how receptive they would be. I know if it came from my best friend or others I respect, I would listen and I also know if I had concerns about something I saw with their kids, they would take what I said seriously, but I also have seen parents where it was clear that the other parents had no interest in thinking something is wrong with their perfect little angel and have lots of excuses for any negative behavior.
If the other parents are one of those difficult ones, you may also need to start considering protecting your own family/home and never allowing this child around your children or animals, or things you treasure - until you can see clear indications that the disturbing behaviors you see are no longer there.
