Get to know him.
It appears that the consensus response so far is "let it go." This is easy to say for outsiders and makes sense at face value.
However, my personal suggestion is somewhat different: Get to know him. Here is why:
It sounds to me like you don't really know this person.
Your reasons for the affection seem to emphasize appearance characteristics rather than qualities of character (actually, it seems that when you describe his qualities, they don't seem all that worthy of adoration).
Appearances can be deceiving, and making any moves without getting to know the person first in a more routine, everyday kind of way would be presumptuous and the results likely to disappoint.
As others have commented, workplace relationships are not a good idea. Of course it always depends, and in some circumstances it might be OK to have a short romantic fling with a colleague if you are not in mutually dependent or competitive roles (e.g. on same team), or if you are about to leave that job (and the relationship) relatively soon, and don't anticipate problems in case the relationship doesn't work out.
I won't speculate whether your task is made more or less complicated by the fact that this person has a girlfriend, as that depends on the girlfriend. But generally, it should complicate things sooner or later, as the nature of love triangles is such that drama is inevitable.
At the same time, the above points do not suggest that the only sensible course of action in this case is to drop it. It is one possible course of action, and perhaps the simplest. But this answer could be applied to just about any situation where there is a hint of a problem with a potential or budding relationship.
My guess is, you would not have taken the time to post this question if you didn't feel like you had something to lose if you let it go. My guess is, you are interested in other options.
Therefore, my recommendation would be to take it very slow, and to begin to know that person better. Try to refocus your interaction with him on work-related matters, then mix in a few comments on more general topics here and there (a casually mentioned hobby, travel plans, likes/dislikes of neutral nature - food, weather, music, movies, travel destinations, things to do on weekends).
Your goal is not to make him like you, but to learn about him, find out what he is about. If attraction is meant to be, it will happen naturally in its due time. If it is not, then this will become apparent to you and you will just as naturally let it go. My sense is that you are just not yet at the point when you can make an education decision one way or another. Good luck!