To clarify:
If he goes to the 6th grade math class during his class' lunch and recess, then he comes back with his lunch and eats in the classroom (or office, he likes to hang with the principal and counselor) while his class is doing spelling, which he doesn't need anyway. Then his class does math, and during that time they have a notion to rotate him among a list of half a dozen options that he needs, such as his "social stories", Read and Respond exercises for his reading comprehension which he struggles with, a writing journal and/or writing the class article each week for the newsletter that goes home, a program they got him last year called Reading Detectives, and something else I forget. Those are all things that they have generally pulled him out for anyway, so that's a good time to do them. Also, they are talking about having him pick a "buddy" to go outside with for some individual recess, as there is no class recess at that point of the day. His 4th grade class will have a recess at 2:30 every afternoon, so he will at least have that.
He will know a lot of the kids in the 6th GT math class, because he went to 5th grade for math last year at the start of the year and occasionally dropped in the rest of the year as well. He may also know some or more of the kids in the 7th grade math class, as he went to 5th grade math two years ago for most of the year. So there should be a reasonable knowledge of him among whichever class, and he gets along fine (better) with the older kids. The only problem we had last year was that some of them thought it was funny trying to get him to ask out a pretty girl in the class. That might get worse in middle school. Urgh.
I just don't know about the 6th grade class, even if it is advanced kids -- it will be fine, I think, for him learning to work within a class and have homework every day and all that, which is what he really needs, but it could be a boring curriculum for him. I think he would be comfortable with the 7th grade curriculum, but that would be problematic if those kids needed more repetition and got him bogged down waiting for them. He told me last night that he was worried about school starting because "everything is so slow, they just keep doing the same things over and over and it's so boring." We're working on understanding that the higher the class level, the more material they have to cover, so they have less and less time to keep working on the same things repeatedly. I hope that is true, anyway, and his 4th grade teacher says they have a lot to get through this year. They do have three cool field trips this fall -- 4th grade goes to Mesa Verde National Park (Anasazi Indian ruins), Crow Canyon Archaeological Park (more Indian Ruins and get to dig in the dirt, overnight trip), and the Anasazi Heritage Center (more Indian ruins, LOL). So they're doing a lot of local history, might keep him busy.