My boyfriend is hanging out with his ex...?

she's won him back 3 times, making him dump his girlfriends and then she dumps him and he gets a new girlfriend and they hate eachother for a bit. Recently he's started hanging with her again. He was suppossed to hang with me and go see fireworks on canada day (i'm canadian btw) and instead he went to her trailer to see fireworks. (he says she made him, but what did she do, hold a gun to his head? hypnotize him?) Anyways, they've been hanging out quite a bit lately and now he's switching to her school! (he says just because he doen'st like the uniforms at our school but he never complained before and we even planned to switch our cles to be together before they started haning out with her again...and why not switch to bayridge and not frontenac. He only knows rebecca at bayridge and he knows may more ppl at frontenac and even more at holy cross!) If I don't do something I'm going to end up his ex girlfriend. Should i tell him not to hang out with his ex anymore? Would you tolerate your boyfriend acting like this? What should I do?