Trying to be maintain a friendship with an ex boyfriend's friend/ best friend's roommate?

So tonight I was invited to have a beer with my best friend’s roommate slash ex boyfriend’s good friend, just to have some good conversation and to get out of the house (friends only). I have kind of known him for a while but not really well. Anyway, when I met him he couldn’t keep eye contact with me and acted distant. It bothered me when he started talking to the other people at the bar instead of engaging in a normal conversation with me, which I thought was lame. I left the bar feeling irritated and that it was a waste of time to meet him. So I don’t know what to think of this situation. Any ideas what he could be thinking? I just want a normal friendship with him.