I have feelings for someone who is in a relationship with someone else?

I have known this guy for about 5 years. When we first met we flirted a lot and there were a few occurrences where we kissed, but we were both pretty young and just out to have a good time. Each of us went to different schools and I settled into a relationship with someone for quite a while, we broke up about 5 months ago. And he started dating a girl about 2 years ago. We both ended up moving back to our hometown, his girlfriend followed him, and they just very recently moved in together. However, before they moved in he mentioned something to his friends about not being sure in his relationship. I recently, unintentionally, started developing feelings for him and confided in a mutual friend about it. Turns out this mutual friend mentioned it to him and basically relayed it back to me that this guy said he likes me and always has. Long story short, we ended up all going out last weekend. His girlfriend left early to go to their house and he stayed out with us. He was really overtly flirtatious with me the whole night, mind you we had both had a few to drink. I ended up giving him a ride home and when we got to his house HE kissed ME. I did not initiate this even slightly, I wouldn't do that on my own. Anyways, I'm not going to sugar coat it - we made out for about 30 minutes. I should have stopped myself, but I didn't. We have communicated since and are keeping things under wraps. He spoke to our mutual friend about it and told him that I was the first girl he had kissed other than his girlfriend in 2 years. However, he also said he thinks we would be good together but that he loves his girlfriend. I don't think he's happy, and I want to be with him. What should I do?