He likes me but he has a girlfriend, what should I tell him?

We've known each other since we were 7. When we were 14 he tried to ask me out, I liked him then too, but I wasn't ready for boys just then... now 8 years later we are back in touch and we really enjoy talking to each other, and basically just catching up. The real problem is he has a girlfriend, whom he claims to be in love with, but he is actively dropping by my place trying to get to spend more time with me. But back to my question... He's a great friend, and I do still care for him (romantically) after all these years, but i'm not ready to set myself up for heartache. I'm just really moving on from a bad breakup, and I couldn't fathom hurting his girlfriend by doing to her what my ex did to me. I could just be selfish and make a move on him, but I could never live with myself for doing something like that... I want to tell him all this without hurting his feelings, and discouraging him from still wanting to be friends with me. How should I do this??? Any suggestions???