sounds like he's given up or something. my boyfriend started acting similar to that when we had our most recent argument. he said he was tired of having to do all the work in our relationship.
and I finally understood what he meant. everytime we would have a fight I would always think of how I was hurting, always me me me. I never bothered to see how it hurt him too...having to put on a happy face and pretend nothing happened when inside he was breaking apart. I was pretty selfish and now were trying to patch things up.
try writing him a letter and ask him nicely what is wrong. if he doesn't want to then tell him your gonna take a break until he decides what it is he wants to do.