It seems like he is flirting with you because he likes you. But you have to think about if you break up with your boyfriend to get with Nathan, will Nathan flirt with other girls when you are with him like he does to his current girlfriend. If it was me, I would tell him to stop flirting with you while he has a girlfriend, and I would stick to the boyfriend I have. As far as you know, your boyfriend that you are with right now could be the guy for you. Nathan could be too, but do you really want him to be flirting with others while you are with him.
You could always talk to him about what is going on. He could just be being friendly and doesnt realize that he is flirting with you. Talking to him is a good way to get to the bottom of everything and then make an educated decision about what you want to do about the situation.
I hope everything works out.
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Thanks everyone for the points. I love you all.