HER boyfriend.?

so here's the dish. ahah. I have a super good friend who happily has all my cles but one. she has a boyfriend. Ive known her boyfriend longer than i have known her, and her boyfriend is probaly my third or fourth bestguyfriend. But it seems like he's constantly crossing the line. he flirts with me, and our realationship has always been like that, but i think it should stop. I have 3 cles with him, 2 with her, one without. our teachers arent helping. i sit by him in every single cl but PE, where we dont have seats. she sits across the room. he texts me all the time, its just insane. The guy i have a crush on likes me back, and when i told him he was actin all jealous. I cant help talking to him, hes my friend, but im almost getting the feeling he likes me. how can i politely tell him to back off subtly, while maintaining our friendship?