Any suggestions?

My boyfriend and I have been together nearly three years. We moved in together at the beginning of the year. few months ago he started acting weird, and wouldn't tell me what's wrong. He asked me to go stay at my parents place, so he could have some space to think. We talked it out, sorta, he never did really tell me what was wrong, but he stayed and I thought we were good and everything was fine and dandy. But now he's doing it again. He says he's unhappy at times yet happy at others. He is not really certain what's wrong. I have offered to work it out and to try and "fix" whatever it is to try and make it work but I don't know what to do. Any advice or suggestions?