How do you get the person you like to be yours if he likes you back but is with someone else?

Play nice. You dont want to get in the way of someone else relationship, that's not the way to start off your own. Just hang in the back ground and see if any opportunities arise where you can make your self noticeable to him. BY the way you want to make him YOURS is he a possession or a obsession...... If he reciprocates your feelings, why has he chosen to stay in his current relationship? No one is forcing him to stay with his girlfriend. Either he's not as interested as you believe, or he's possibly wanting to cheat on his girlfriend with you. I'd be wary of him.