Ex boyfriend problems 😖🤔

Ok so I'm not sure what to do. My ex and I were getting close to getting back together in February (Valentine's weekend), and we talked for a while then he stopped talking to me and then got with his ex. But now he keeps flirting with me on and off and I think he knows I like him again but I'm not sure what to do since he has a gf. But me and him have been there for each other no matter what for the past 5 years. I don't know what to do anyone have advice ???? (Btw I'm 17 about to be 18 and he's about to turn 18 next week)