Why is he acting weirdly around me for a long time?

Hi, so I was really close to my best guy friend for two years and then when we both moved to upper school he has been acting differently and ignoring me a bit. We were really close and everything and we used to play fight, joke around and all that sort of stuff. He was hanging round with this other girl who people say acts and looks exactly like me and she has been flirting with him a lot.she does the whole hair twirling and ruffling his hair kind of thing. Everyone says that she flirts but she denies it straight away. He looks really uncomfortable when she flirts with him though. Like when she flips his hair he like stops her and puts it back into place, as if she never did it. His other friend told me that he definitely dosen't fancy her because she wears too much make up. When he tells a lot of jokes, he always looks at me to see if I am laughing and checks around 3 or 4 times. When i joke around with my other friends he always laughs wierdly and then when I look at him sometimes his eyes dart everywhere. I asked a question on yahoo answers before about this ( it was more detailed) and it had mixed results and the best answer said that he might like me ( I'm not so sure?) and I should start talking to him more. I don't know what to say though, I am afraid that the girl with get jealous or if he will act even more weirdly around me. What is good to talk about in a conversation with him? Thx for taking the time and patience to answer and read my question?