Have you already started some type of relationship with this guy? If you have or are thinking of starting one, then you probably should talk with him first about the type of relationship you are looking for.

I think it is reasonable for you to be concerned about his ex-girlfriend, but I think you might not want to jump to conclusions too quickly. Have you let him know that it hurts you to hear they're hanging out? It could help to ask him questions so that you can better understand their relationship and whether or not the rumors are true.

Maybe he still feels close to his ex-girlfriend and wants to remain friends with her. All breakups are different, and it could be that their friendship is still important to him. Many people do maintain relationships with exes, but it doesn't always mean they are "not over them." But if he is in fact still in contact, it could also mean that he isn't sure what he wants. Perhaps he needs more time before he is ready to start a new relationship.

Whatever the nature of his relationship with his ex-girlfriend, you still need to decide for yourself what will make you happy. You might even want to wait a little bit and give him some time before jumping into something new.