maybe he is at the "crossroad" of his life ... he loves his girlfriend but he likes you as well, this is difficult...
give it time. maybe he no longer responds to you because he does not know what to do or say yet
but if you get to see or speak to him next time and still ignores you then maybe he does not really have the courage or decency to tell you about his emotion / decision
Maybe he wasnt trying to ignore you but his gf knowing how close you are felt a little jealous and asked him to stop talking to you as much. It sucks but how would you feel in her position that you loved someone who was always talking to another girl.
Anys ways I think you should go find him and let everything in the open. If it goes good then great and if not then whats the difference you werent talking much anyways. I was in love with a guy who I was friends with for a long time and never had the courage to tell him and he and I went no where. Youll never get anywhere if you never take a step. Be spontaneous and go for it, dont hold back cause you may never get the chance again.
Tell him how you feel the first chance you get...dont let it slip by...i have been there and regret NOT telling her!!!