There is this boy at school i really like but he already has a gf who is one of my mates,what shall i do?

I asked him out a while ago when he was single and he said no sorry. But ever since then he has been flirting with me alot cos i sit next to him alot in school. Then i went on holiday and when i came back he was gong out with my mate. He still flirts with me alot and not with anyone else. I wasnt here for a day last week and on that day he seemed really distant toward her and he keeps on staring at me and its too hard to move on!