You saying you're best friends with an ex-boyfriend adds an entirely different dynamic to this. Regardless of what you say, the feelings you are expressing here are the same ones he feels when you are hanging out alone with an ex-boyfriend that you've more likely than not been sexually active with. You can't expect him to act differently towards this girl unless you act differently towards that guy (i.e. not hanging out with him at all or only when your current boyfriend is present). Once again, this is something that needs a serious discussion between the two of you.