Side chick

Okay so there's a guy at my school who is in a on and off relationship with this girl for a year. For the past 2 months me and him have been messing around and I've started to catch feels. I don't want to be a second choice but it's so hard because when we are together it's just me and him and it's amazing. He is in a relationship with the same girl now but says it will be over soon. What do I do?