Why is he all of a sudden interested in me?

I think you should follow your gut feeling. Yes by reading your story it does seem he wants to be more then just a friend and it does not seem he wants a relationship either since he knows you have a boyfriend and he is still trying to pursue you to hang out with him one on one. A good way to find out his intentions for sure and not ruin your friendship if you are not sure if his motives are inocent or not is to bring others to the scene like your boyfriend or other friends. Tell him sure we can hang out with you, and tell him this person and that person are coming too. You will find out his reaction and get your answer for sure as he is most likely going to back off.