I'd say that it depends.
At that age, my DD was able to do some self-advocacy with adults, but it was a little erratic, and sometimes she would shut down and become mute due to anxiety if she felt that the conversation was going nowhere.
Then again, I also found that when she advocated, it was generally far more effective with adults who figured that she was the product of helicopter parenting run amok.
I might be inclined to let him try, in light of what you've posted-- give him YOUR availability, and tell him to set up a meeting with the principal at a time when you can go along as the silent partner in the room. I would probably NOT want him to go into such a meeting alone, no matter how good his social skills seem for his age. I wouldn't have even with my DD, and she's something like a social savant.
She definitely was running her own advocacy show with her 504 meetings by age 12 or so, however-- and some of those meetings were anything but easy or cordial. IMMV.
