If he has a girlfriend at this point in time, you need to respect that. If you attempt to get him back while he is with said girlfriend, who is to say that he wouldn't do the same to you if you got back together, and another girl wanted to be with him?
Karma always comes back around. Do unto others as you want done unto you.
You can be his friend, but you should only be his friend if you can remain platonic. If anything, not showing interest in getting back together may spark his interest in getting back together. [This is the whole "the grass in greener" thing!] Good luck!