Do you WANT to keep him?
If the answer is right away "yes" then try another trainer. Sometimes some people click and other people don't.
If the answer is "ya, but..." then sell. No sense in pouring out money, time, spirit and love if you feel that you are going nowhere. Sounds like you've put quite a bit of effort into him.
Afterthought: have you contacted his breeder or previous owner? Maybe they would have some insight.