Most Helpful Guy
you can talk to him about it, let him know whats on your mind.
let him know how you feel and that having this insecurity isn't maing you happy.
if he wants his ex and is not ready for a different relationship, i'd say its time to move on, how much that might suck, only thing you can do.
Most Helpful Girl
Leave him for sure, you don't deserve to be second place! Why is he with you if he isn't over her yet? He's clearly not ready.