You should think about this...

If you kept him in fifth grade, he'd have the social models, he wouldn't have to change classes. What kind of "social promotion" is it if they "promoted" him from typical fifth graders to a self-contained room with no social models? What disadvantage is it to hold him back a year or two? They have to legally educate him to age 21 - two or three years beyond the other kids. If you think peer models are the most important thing he can be getting at school (not that I necessarily agree...), here's a way to have the peer models. Whether they are 5th or 6th graders, they are still modelling behavior years ahead of his development.

OK, I tried...