hmm.... Interesting. So he has GF, but plays with you. Maybe it's just harmless game or he is unhappy with current relationship or (that would I behave
) OK with current relationship but honing his seduction skill. And probably successfully.
I don't think question is what HE's up to. Question is what you want. Do you want to take a risk dealing with guy with another girl? (I am not saying yes or no. Just asking). Simply answer to yourself.
To your second question, it's very easy to overcome this game a) play along. If you are "dear" he's "honey". That will confuse him. Imitate all his tricks. Also, make sure you have other interesting males around you so you are not stuck with him in your thoughts all day long. You need people who will provide you with similar emotional swig as he does. So you are not attached to him.
Does that answer your question?