Just because he’s with someone else doesn’t mean he doesn’t like or love you. People can love more than one person. Relationships aren’t just about love. They are also about circumstances.
You sound young, so maybe you don’t know what I’m about to say. When you first experience “love,” it can seem overwhelming. You can obsess about your object of love. It feels like love because that’s all you seem to be thinking about. Also, if your love object is taken, you usually become even more attracted.
How do you win him over? Well, one thing is for sure, you shouldn’t make it too easy. If so, he may just take advantage of you without really caring about you. You should be thankful that he thinks well enough of you that instead of taking advantage of you, he wants to remain friends.
At your age (if I am right about your age), relationships often do not last. Be patient. If you still love him when he breaks up with his current flame, you may well get your chance—if you still feel that way about him. Of course, you may be involved with someone else by that time, but don’t worry. That will make you even more attractive.
Of course, time heals all wounds. The longer he does not seem to be interested in you, the more you will “get over” it. On the other hand, if he keeps maintaining a lot of face time with you, that should help you stay interested—maybe not so good for you, but good for him, and maybe good for you in the long run.
In any case, mistrust this feeling that you are completely in love with him. You don’t know him well enough if you haven’t been in a relationship with him. You have a crush on him. Crushes can turn into love relationships, if nurtured properly, but they will wilt and die without good soil to grow in, and a decent amount of water.
Be patient. It is not over yet. He may move on to you. You may move on to someone else, and no longer be interested in him. I’m sure it all seems overwhelmingly important right now, but in a few years, you will look back, and with the experience you have then, you will see this in a very different light.