If he's really monogamous you are in trouble....especially if this girl is mongamous as well...I'd say the writing is on the wall in that case. Why would he deal with jealousy and you having other partners if he doesn't have to? She'd be a a much easier partner to be with as far as intimate comittment goes. She could be a total head case in other areas though.
If he's not mongamous, you'll have to deal with his interests as poly is a two way street.
Move forward cautiously would be my advice...don't invest to heavily until you see what is really transpiring. If he is forcing himself to be less than monogamous just to take the sting out of you being with other guys, again you are heading into trouble.
Lots of work to do I'm afraid. Good luck my and stay healthy.
Playing the Game of Life with Monopoly rules.
Monogamy might just be in my genes
Poly Events All Over