Should I tell my kind-of-boyfriend that I had with his good friend?

I've had a fling with this guy since May, we were pretty much together but not officially since he was leaving for college in August and we didn't want to have anything serious. We really did care about each other though, and we hung out (and hooked up) all the time over summer. After he left, we haven't been talking much, just random short calls, texts, and IMs here and there. Yesterday night, my friend had a huge party and I ended up having with a guy I'm comfortable with. The only problem is that he's pretty good friends with my kind-of-boyfriend. I know that if I told him, he'd probably say he doesn't care but would actually be a little hurt inside. And we're friends with a lot of the same people so I feel like he'll hear about it eventually. Should I tell him first? And if so, how should I bring it up? No mean comments please. Thanks in advance!