The question before asking HOW you can get him to leave is girlfriend is … WHY do you want them to split up in the first place? Consider several things before you try to make a move.
Assess the situation.
You don't want to be a home-wrecker, or relationship breaker no matter what your intentions are. So weigh your options carefully. Think of it this way, if you're in her shoes, you wouldn't want anyone to meddle, would you?
Talk to him
If he's a friend and you're sincerely concerned, then talk to him. Let him know that you're giving this unsolicited advice because you care for him as a friend. Then drop the subject and don't push it. Give him time to consider and let him make the choice.
If you simply want him for yourself, then don't cling
Clinging and being desperate wouldn't make him want to choose you over her. In fact, if he senses that you are trying to tie a tight leash around his neck, chances are he would avoid being caught. Let him go and move on with your life.
Don't threaten him in any way
Don't make him choose on the spot who he's going to keep, you or his girlfriend – because chances are you would get hurt. And don't say nasty things about her, either, even if he does.
Wait for your turn
If he says he intends to break up with her anyway, then be patient. Don't get into any kind of serious relationship with him until he does so. And don't be a friend with benefits either. If he can have both of you at the same time, then you can't expect him to give one or the other up.
Have a life and spend it well
Be confident, go out and make other friends. This is not to make him jealous, this is to open yourself up to other options and to let others to find you.
Let it go-willingly
Don't let your world revolve around him and don't obsess yourself with the idea that he's finally going to give her up. If, once and for all, you two get into a serious discussion about the situation, then simply tell him how you feel. Then let it go and move on with your life.
If he truly values you, he will break up with her and go after you. Otherwise, if he sticks with her, then he's not truly interested in you in the first place.