I think you should go no contact with him. The more you pursue him the more it will push him away.
You can't know what his reasons are so try not to make yourself crazy thinking it over. (I know easier said than done). It could be that he found a new girl, maybe he thinks he is doing you a favor, because he just discovered and STD.. etc etc.. who knows??
The main thing here is that when someone wants to break up you can't be afraid to walk away. Respect yourself and honestly just move on. If he knows he lost a good thing, he will find you. Otherwise, never look back.
Edit: I just realized I replied to a post from 2011. I am sorry to admins and others. Hopefully my advice will help someone in a similar situation though.