I would not leave him with the little ones yet. I don't let my little ducks out with the grown ups (except their mother) until they are nearly full size, usually at least 4 weeks, but more often 6 weeks. Be patient, protect your babies. What you do have going for you is his age - he's a teenager and may well settle down some. However, he's going to be a real pain next spring during breeding season. Be prepared. If he gets along well with one of the girls, you could give him a girlfriend in his pen, then he'd be quite happy. I wouldn't hatch eggs from him though, temperament is pretty hereditary and you don't want to be breeding for aggressive ducks! So, if he breeds a duck, she could have his babies for 3 weeks after! Duck sperm live a long time in the girls.
I'm glad you have a pen for him. Good luck!