Given that he's asking for the change and that he's socially accepted now, it seems the school is trying to "borrow trouble" where there likely is none. Could you or the school find some anecdotes that say otherwise? Sure. Same the other way.
I'd look at what the Belin Blank Center has in the way of reading material on this topic (Nation Deceived and Nation at Risk).
Your child is already mostly in the upper grade. I'd think it far simpler and happier for all to let him be fully in fifth. That stops making him a special case and lets him just be in his grade all the time.
FWIW (anecdotes of two), we have one 10 YO child who has been "half skipping" (two subjects, two grades up) for three years now. He doesn't want to leave his age friends, thus the reason to continue the status quo. YDS was subject accelerated one year during the start of the year and the teachers came to the recommendation it would be better to move him up fully. It is going quite well.
