You said he is so loyal although he looks at you with the eyes of a lover seems to be a contradictory statement. If he was loyal to his girlfriend, then he would not be looking at you with the eyes of a lover. He might not actually cheat on his girlfriend, so in that sense, he is acting loyal towards her, but if he is looking at you in a lustful/loving way, then he is not truly being loyal to her.
I would tread very carefully in this situation. It sounds like you are still in love with him and you are hoping that he is still in love with you. Is it possible that you misread his look? If not, then I would suggest that you steer clear of him. Why would you want a guy that would be giving his ex lustful/loving looks? How would you feel if you were the current girlfriend and you knew that your boyfriend was giving his ex those kinds of looks. I’d be embarrassed and hurt.
I’m guessing that you two broke up for a good reason. Try to recall the reason(s).
If this fellow still loves you, do not persue him at all. Let him make the decision to break up with his current girlfriend before you make any decision to get back together with him. Anything less, would make both you and he both to be really crappy people.
You may or may not know his current girfriend, but her feelings need to be considered in this situation.
If you are truly mis-reading this fellow’s look, and he was just smiling at you as a friendly gesture, then just be polite and friendly back. And be polite and friendly to his new girlfriend too.