Can you define my relationship with this guy?

It appears to me like he just wants a little fun. He may have agirlfriend but he still flirts with others to keep it interesting. He may not even flirt, it may seem like that to you but he may just joke around with you. Whenever I have a boyfriend I joke around with outher too a bit. Nothing serious. He has a girlfriend so I wouldn't develop feelings for this guy. You seem to have a great friendship with him. I wouldn't risk anything. If he does flirst outragously with you, ask him where you stand and what he wants out of all this.