Hi Ray
From what you have said, it seems like he is interested in you. If he was not, he would not have found it necessary to make sure you understood that the woman you saw him with was not someone he wanted to be with or was interested in.
I think you should let him know how you feel... not by telling him but rather by continuing to flirt with him. Just because you are interested in him, does not mean that you have to get into a relationship with him.
Let him know that you enjoy his company and would like to get to know him better....and sing many more duets with him.....
Enjoy