I'm pretty new here myself (lurked for a couple months) and I think you're probably looking for this board: http://www.polyamory.com/forum/forumdisplay.php?f=4
Suffice to say that's a very sad situation to be in, my heart goes out to you. My gut instinct on the matter is it's clear he gets either swept up with or interested in women besides the one he's with and that won't change. Words are wind and actions speak truth. It sounds like you're both not having the same relationship.
In a true poly situation to my understanding, all parties (even those who are mono) consent and are aware of what's going on to the level of their comfort.
Cheating is not poly, it's a destructive behavior that breeds mistrust and without trust ANY relationship falls apart. If you think you could handle it with him dating around, though you could discuss some ground rules to start, or you may just both want different things, but either way he should be completely honest about what he wants in his relationships and so should you.