Most Helpful Guy
Have a calm, direct matter of fact conversation. Tell him you propose there be no further mention of exes. Just ask him point blank if he's considering going back to her. If he says no, get him to agree that if he changes his mind, he'll tell you right away., Then get on with life.
Most Helpful Girl
If that's something you seriously wonder, then it's best for yourself to not invest yourself into anything with him.
Exes can have a magnetic pull which is why it's best not to get involved with someone who just got out of a relationship. They need time to heal in a healthy way where another person isn't being used as a distraction. Plus it's not fair for you to be building a new relationship on something like that.