well this is how I see it no matter what we do we are all still going to have feelings for our ex's it's perfectly understandable but if he is still flirting with her then Yeah there is a bit of a red flag there. just tell him how you feel if he really cares about you then he'll understand. but if he keeps pulling that kind of BS I would say get rid of him. don't waste your time, don't put your life on hold for someone who dosen't give a damn.