What would you do?

So my girlfriends and I have been debating about this topic And I'm interested in what the women of glow think.  
Scenario: You've been exclusively non exclusively dating a guy for a long time. You've had internal trust issues with this guy, but you've recently talked about them. Now the relationship is finally on the verge of commitment. You're discussing moving forward and taking the relationship to the next level and he's telling you how much he wants you in his life and doesn't want anybody else or anybody else to have you. You don't really say much because you don't know if he's serious or not. All of a sudden, a female friend whom he used to have sex with years ago pops up and occasionally flirts with him. He doesn't really pay her any mind but he doesn't really stop it either. 
Would you question him about it and give him the chance to explain ? Leave him alone because he's probably lying and still messing with the girl? Still give the guy a chance but begin dating other guys to keep your options opened ? Show confidence and prove to the guy you're a better catch than she is ??