I think most people just say that he needs to find something to do (a hobby, dating for himself, friends to hang out with during your dates, etc). My husband usually watches the anime he likes, since I don't enjoy it. Or goes to a gaming group. Or works on his woodworking. He really tries to either watch something I don't enjoy or leave the house, though, so that it feels like he is kind of benefiting from me being out since he gets to do something that I don't enjoy.
Dating for a married, poly guy seems to be really difficult, though. In the year since we stuck our toes in the poly waters, I've gone out with about 8 different people (at least for the first date, only 3 have led to subsequent dates) while he has had 1 woman actually go on a date with him. His crazy schedule doesn't help, but realistically it just seems to be harder for men.
I would recommend him joining some groups and trying to meet new people with similar interests. Then as he gets to know them, he may feel comfortable coming out as poly and may find some prospective dates that way. Just an idea.