I'm not sure that I'll add much that others haven't but we do have experience with language arts subject acceleration and grade acceleration for dd12. She went to an accelerated 4th grade grouping for the entire language arts block (reading, writing, spelling) in 3rd and skipped the entire year of 5th.
Because she wasn't the only 3rd grader going to the 4th grade class, that one was easy and she didn't feel like she stood out. In 6th grade she stood out a little b/c she hadn't hit that big growth spurt yet and was one of the smallest in her grade (which was new for her b/c she really isn't unusually tiny like her younger sister who is used to being the smallest). She still is much less physically developed than the other 8th grade girls, but she's grown 8-9 inches in the past two and a half years so she no longer stands out as a shrimp.
Like others have said, there is sometimes a degree of "wow, you're so smart" type of attention from the others kids, but we've found that it abated over time. The other kids, and even the teachers, don't really think about her as younger anymore; she's just one of them.
While they can send him higher level books into K as they seem to be doing, I do like the idea of having the opportunity for peer interactions and discussions about books that likely won't be taking place w/out the acceleration. Dd12's 2nd grade class had a lot of advanced readers in it and it the "high" reading group did have some good conversations about the books they were reading.
