Is he trying to hook up again or what?

Yes, he may have calculated that you would feel more compelled to go by inviting your friend, but there are probably a few possible motivations. He may want to hook up with you again to see how much he can assert himself on you. Unless he is the master of concealment and living a double life, one might believe you didn't really know him that well when you consented to hooking up previously without having a committed relationship. In hooking up somewhat blind to other parts of his life, you set a fairly low bar for him to step over and even though you bucked him when you learned he had a girlfriend, he may still have the impression you might be eased back into messing around.

He's a guy. Guys like to feel powerful and verile. You were a discrete but available channel for those feelings so yes, obviously he would like to hook up again. I doubt he's out benevolently trying to build interest in performance art with the general public...

If you find yourself avoiding him with anxiety and he feels confident in approaching you and getting your close friend's cell number, you need to assess just how you left things. Moreover, you need to decide what you want to be. If you just enjoy being that primal channel for him or other guys without deep connection or commitment, should you really be upset with him for not being exclusive for you? Is it reasonable that you should suddenly take a moral stand? Casual sex is just that. Either celebrate the free spirit you want to be without judging others or calmly commit to becoming another kind of person, learning form your experience without spite.