Do the accelerated kids find that some topics are meeting their needs but others just won't be able to?
My DD is not nearly as accelerated (or in need of acceleration) as your son, and we're only a week into the grade skip (1st to 3rd), but it seems like her asynchronicity has helped her find a balance early in the year, but if she has a growth spurt in one of the lagging areas, it makes the end of the year harder.
So for K, she was ideally placed in K - she couldn't read on entry, was young for grade and not socially mature compared to her older classmates, and did not have great gross motor skills. Maybe slightly ahead in math and fine motor, but not so much that she disenjoyed K. She had a huge growth spurt in reading, a slightly smaller one in maturity, and ended K enough ahead that we contemplated acceleration, but decided she didn't need it.
For first, she had an OK placement at the beginning of the year, but had a huge growth spurt in math, gross motor, and maturity, and really started complaining about being bored. Her 1st grade teacher asked us if we thought her placement in first had been a mistake, and it totally wasn't - she needed that year. But really, she mostly needed the first 4-6 months of it.
So far, third seems to be OK. She anticipated being behind in math, but they're starting with such basic review, and she catches on so fast, that I think she's going to start and stay towards the top. Her reading has historically been above her math, but wasn't so much an interest last year - if she has a big reading spurt this year, I could see her being way ahead in reading, although she's high average now. Her fine motor is good enough to keep up with the increased writing load and introduction to cursive. But her writing isn't fast enough that she finishes her work early, so that's the lagging area that's keeping school interesting for her at the moment.
Second would have been a bad placement for her. And fourth would have been an impossible placement for her. Although, having seen how third is starting, I think she might have been able to rise to the occasion and do the work in a 4th grade class, with sufficient support and motivation.
