How can I know if my boyfriend is really totally happy with me?

My boyfriend and I have been together for 5 months, but we've known each other for 4 years. For the pass month I've been getting this feeling as if he really isn't happy. Every time I ask him if everything is ok, he just answers me with a smile and says "YUP, EVERYTHING'S OK?" But something deep in side of me is telling me that he's not actually being truthful. I don't know what to do.