Well, if a boy doesn't like you, he doesn't like you. Sometimes there is nothing you can do about it. If you guys are friends, just talk to him but don't tell him you like him until he gives a hint...
This boy obviously liked something about you. Have you changed in anyway? fell out with any of his mates? maybe he just moved on. How long ago did he like you? if it wasnt that long ago, you can...
look around and see if there are any other boys you might be interested in, if you chase or act desperate he will run even farther... plus if he acts like that you should be looking at a different...
Become really close friends with him so you can show off all your best qualities, some of the best relationships start out this way. It is better to become friends first.