How to reassure an insecure boyfriend?

Alright this totally got me out of the blue.

Sunday night I was out with my boyfriend and a couple of mates and this random guy sorta hit on me but I just blew it off. My boyfriend got a little possessive and draped his arms over me and said something smart back to the guy.

Later on that night my boyfriend started going on about how he couldn't imagine that I hadn't cheated on him (in a sad emotional way) and that if I wanted I could have any guy in the bar.

I don't understand where this all came from and he genuinely seemed down and out about it.

Is there anyway to reassure him that it's all in his head and that I only want him?