i have known this guy for almost 2 months now, when we met he was with someone else but it was clear that the relationship was already near the end by the time i met him. they recently broke up and he and I are unofficially together. I am terrified of being the rebound woman. he assures me this is not the case but his ex keeps calling him all the time and he usually answers the phone and listens to her crying and telling him what a bastard he is

.
he is wonderful and we get on so well. we really do have strong feelings for one another.
i know it took me ages to get over my ex. i don't want to let this wonderful guy slip through my fingers but i also don't want to doom our relationship by hoping for too much too soon and ending up his rebound relationship.....
any advice would be most welcome.