This is going to sound cruel, and you can hate me for it if you want, but I am telling you now that I was with this girl for like six years and I started hangin out with someone else, and it developed into something more than friendship. I too did not know what to do, as I loved my girl deeply, but the thought of something new was overpowering. It got to the point that I was forced to make a decision, so I decided to leave them both. I still have nighmares about the look in my girls eyes when I told her about it all. Thats what you have to do, you have to force him to make a decision cause if you don't, he will drag you along forever if you let him.
If he can't live without u, then his choice should be very easy.