I would say stop messing with him. He's an idiot for asking you in that way and in that situation (right after you hooked up). If he wanted to tell you about the other girl, he could have just been upfront about and not ask in such a stupid way. So do you want to be with someone that stupid?
And do you want to mess with someone that is starting a new relationship? That would just be messed up for the other girl.
I'm sure there are other guys out there that are better (in bed) than him
