Will he leave his girlfriend

im going through the same thing what I would do is just wait until he is done dating her and then tell him how you feel and when he brakes up with her or she brakes up with him you can be there to help him with that. also you dont want to tell him when he is in a relationship because the other girl will be like he dumped me for her and then she will say stuff about you that you dont want when their is a lot of other stuff you can have at that time that needs to be done. just ask him for his number ( thats if you dont have it yet) and yet till they are not dating anymore.

Dont ever assume anything. Also, he has a girlfriend, so who are you to go and break up their relationship. Get some space from him and let him make his own decision. How would you feel if you were that other girl? You would be pissed. Have some respect for yourself and for her, and back off. He needs to decide what to do, and you dont need to be influencing that decision.