It is normal for your boyfriend to not be up to talking about his ex. You might think that he was in a committed relationship and will want to talk about it, but his feelings for her are likely still fresh in his mind. He might be thinking back on the happy memories of when the two were together and be sad that they aren't seeing each other anymore. When people are going through a divorce or break up, they often avoid talking about it because it can bring up mixed emotions.
He may not want to bring up the past for any number of different reasons. Maybe he knows that you don't like his ex and isn't sure if you would accept him dating her. Or maybe he just isn't ready to talk about it yet.
However, if he is avoiding talking about the relationship and getting more and more distant as time goes by, then there may be something wrong with it. It is important to know that someone has a problem with their ex before going out with them when they have unresolved feelings over them being together in the first place.
He may not want to admit that he still has strong feelings for her, but she has had an influence on him. A good thing to do is to find out from him why he doesn't want to talk about her. You should be able to come up with a solution that will work for both of you. For example, if she was very beautiful and intelligent, you can say that you understand why he may be attracted to her but you are a completely different person.
If he isn't ready, then tell him that you can wait until whenever he gets the time and feels comfortable talking about it. Just make sure that your boyfriend isn't allowing his ex or past relationship to get in the way of your future together.
