You can't "Get" someone to like you, but you can get them to know you better. When you see more sides of a person things get a lot more interesting. Just try and hang out more, get to talking if he's down for a chat. Ultimately, if he likes you back, he'll probably express the interest.
Simply, talk to him everyday so that he can get used to talking to you. He'll probably want to talk to you more. Or ask for his number or you give him your number. Tell him to text you stuff like that. Just don't make him break up with his girlfriend because that isn't good.
You can't force someone to like you. If they like you, they like you.
However, he's taken so back off. Do you really want to be "the other woman"? How would you feel if you were dating him and some other girl is vying for his attention and in the end he leaves YOU for HER?