How to tell 12 year old he is high functioning autism?

My 12 year old son is struggling with friend ships in high school. He at times, does not get it, whatever it is!  We have never disclosed his diagnosis, not because we were ashamed, but because he can muddle through and not Be labelled. 

No he is in high school, things are slightly different. He sit's down at lunch, and others get up and leave.  He seems to float from one group of boys, to another, never really fitting in, and he doesn't understand why. 

We are thinking about telling him of his diagnosis, in the hope it explains things a bit, but I don't know where to start. Anyone any good info, woul be great!