what the fuck is he thinking?

So there is this guy I see one weekend a month. He is very attractive and about 1 year ago we ended up having sex. We didn't talk much after that encounter but recently we started talking again and the other night we kissed and I ended up giving him a blow job. Afterwards we talked and he told me he had a girlfriend that he's been dating for almost a year and that they may move in together and said we should keep this situation between us. So I said well next time you're single let me know and he said he definitely will. And brought up how we have to spend 3 weeks with each other and who knows what will happen then. I thought we came to an agreement there is nothing necessarily between us but when ever he sees me he gives me this intense stare. Eyes wide and it looks like he's deep in thought. I catch him doing it and stare at him back for a minute and then add a lip bite or a smile before he or I look away. 
What does this mean? What could he possibly be thinking?