feel totally uncomfortable at first, but trust him,
If they are best friends with their ex, that means I knew it when we got involved... and I'd be fine with it.
I AM that guy. Believe me, my wife's got nothing to worry about. There's a whole bunch of good reasons we're no longer a couple. If I decided to screw around, it wouldn't be with my ex.
Get to know the girl, and make sure I communicated any feelings of insecurity or uncomfortableness with my boyfriend. Honesty is key. :)
Everybody is everybody's ex-boy-girl-friend in a small town. My country is loaded with these scenarios.
Make your feelings/ thoughts known to him...Though personally I don't think it would be ideal to stay with him when his bf is his ex!