I feel like I was in a similar situation with you, well the guy, anyway (I gave details on my
How to fall out of love thread).
If I was the guy in your situation, I’d probably be thinking that although he really liked you, he didn’t want to feel like a complete loner because you were already in a relationship, to show that it wasn’t a big deal or something. Is the person(s) he is seeing now serious? Has he already moved on?
Are you still in this other relationship? Do you have serious feelings for this guy you are talking about? Or are you just trying to distance yourself from him? Not quite sure what you want. The best thing I can say is probably to be direct with him and clear the air about everything that is going on.
Best of luck to you