He says I'm the highlight of his day but has a girlfriend?

I know this guy for about 1,5 years, we work together but not in the same group. He acts very protective over me when other guys hit on me, compliments me that im pretty and attractive. Nowadays he comes to my desk every day and we talk and eat chocolate together. We are friendly with each other, but both pf us are attracted to one another. I just got promoted and he wants to get my previous job, and be at the same office as me (it'd be the 3 of us). I would be the one tutoring him. He just said that im the highlight of his day, but he has a girlfriend who he never mentions. What should I do? he's a nice guy, but this just kills me...
He says I'm the highlight of his day but has a girlfriend?
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