Most Helpful Guy
I think that it is obvious that he likes you. Out of respect to his relationship he may be distancing himself from you as to not start anything and complicate his current situation.
People find other people attractive all the time, it is hard not to interact with attractive people in the workplace especially if you have chemistry. I don't think that he is particularly a "scumbag" as some may think but more so just scared you would dismiss him immediately if he had mentioned his girlfriend.
Keep it friendly with him, not that you would but don't cross any boundaries and who knows maybe somewhere down the line he will be single after all.