When a guy says "I'm thinking about you", I miss you, I like you, etc., he is trying to get your attention, to see if you might start "thinking about him". When he said "what about you?" He means are YOU available? Maybe YOU could be the girlfriend he should "get". Yes, he is "flirting" with you, and he will continue if you keep talking to him. As long as you talk to him, he has your attention, so you have to decide to "go silent" or keep up the dialogue.