Well, you have a roadblock to everything

Fifth grade, sixth grade - there really isn't much difference. It's one year - but it's a totally different school setting - one that you thought beneficial, vs. the one he's in now that you consider harmful.

I think children with disabilities should be grouped by their developmental level. The key is getting to the next level. If it takes 30 years to get to a 10 year old level, so what? An adult at a 10 year old level can live semi-independently. Chronological age is meaningless for kids with serious disabilities. Social promotion is so much less important than a good program. In fact, I'd go as far as to say that your son shouldn't be in a "grade." He's entitled to a public education until age 21. When he finishes 12th grade at 18, what grade will he be in at 19?

I held Emily back a year and it's the best thing I ever did. A year for your son wouldn't matter as far as peer group, but it's a dramatic difference in program.