I think the guy I made out with has a girlfriend
Oh no! Why would you make out with someone that has a girlfriend? Why do you think this?
All the time when he's with me he seems a bit uncomfortable
I hope you're wrong, because it's bad to make out with someone who's in a relationship!  It's even worse that he was cheating on his girlfriend, assuming he had one.  I'd get to the bottom of this as soon as possible!
I'm not sure how to do it. It's not like I want the guy to tell me about his relationships. I just can't help getting wayyyy too emotional about things like this.
You should try to find out what is going on with his other relationships. Don't you think you deserve to be in a relationship that is built on trust?