How can i get him back?

since you are good Friends so you probably must be knowing many things about him.... both that are known to the world and those that aren't ...... so try to get closer to him by asking about all these things in general at first ..... after a few days talk.... focus on more specific topics like if he facing any kindda problem with a colleague or a professor or even his parents and take a regular follow up of those ...... but in a casual manner..... in this way show to him that you still care bout him....and no matter how much you get involved with any guy in the world you'll always have some time for him and his problems...... if you manage to impress him this way then may be he might get back to you....... dnt try to become his gal.... try to be his best friend and one day may get a surprised to know that you have become more than a best friend to him.....