How to get a 15 yr old to dump his 4 month girlfriend?

Theres this 15 yr old tht likes me nd we txt all the time.But he has a gf nd they've been dating for four months.but he still asks me to sneek out with him nd we always flirt around nd tlk abt sex.i liked a shirtless pic of him on face book nd his girlfriend yelled at me.wat do i do i still wanna flirt with him but not wen he has a gf. i rlly like him.............