I have dealt with this and my feeling is that you are not going to get anywhere. Even if you somehow get him to cave, he is going to make sure that he is proven right, and look for all the negatives. When he starts crying in school it will be, "See, I told you he was too immature and you wouldn't listen." (rather than trying to help solve the problem) KWIM? We had the option of switching schools and getting into a better environment, which solved our problem, but that's not an option for everyone, so you may have to just make the best of it.
This kind of thing may happen no matter what you do as parents, by the way.
I've decided that it comes with the territory of having a PG child. Whatever you do, rest assured that Monday morning quarterbacking will make it out to be profoundly wrong.

Sometimes this occurs on a level which is downright crazy, to be perfectly honest. I mean, if your child is robbed or their academic program disappears under them, being emotionally upset would be pretty normal at either 15
or at 21-- but you can be sure that them being 15 will be a thing for some people anyway.
It makes me wonder what it's about for those people who
aren't accelerated-- because they don't have that handy excuse of "immaturity" to blame for every thing that doesn't look just like
Pleasantville or Stepford. I've often wanted to ask what they would blame instead-- though I don't have sufficient cheek in the moment, I fear.
~Mom to a successful and accelerated college student.