Is he just being polite? What's his deal? Just can't figure him out!

Okay so this guy at work knows I am attracted to him, he said he saw me as a friendly colleague, but over the past 8 months his behavior around me has changed a lot. We never really even noticed each other for the first 2 years, we work in diff areas of the building, but we do have a mutual friend.

Since he found out I like him he flits between giving me lots of prolonged eye contact, to then ignoring me a few days. When I walk in the staffroom his eyes meet mine, and he tracks where I go with his eyes. He always sits within my direct view, when he's busy talking to someone, he will break eye contact with them and gaze at me as he continues the conversation, to the point that they stop and look at me also. Initially he was very helpful towards me but this has subsided. He is very outgoing with his friends at work, but around me he is reserved, careful what he says to me and laughs at everything I say, even if it's not that amusing. When I sit next to him he goes silent but doesn't move, like he enjoys my physical closeness, but he never touches me. Always stands within my personal space and tilts his head when he listens to me, like he's drawing me in for a kiss with no follow through. After he found out I liked him his clothes style changed-shirts and ties really smart. Would stroke his tie when I approached him. Our mutual friend said he is just being polite and has no interest in me, but all of his body language suggests he is, but she also said he has a girlfriend, although when ever anyone mentions her in front of me he changes the subject and looks straight at me. What's this guys deal? I really like him a lot but I am utterly confused.

Is he just being polite? What's his deal? Just can't figure him out!
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