Welcome to effiekins and charlottemom!
Effiekins, there are quite a few book threads on this forum, if you do some searches. A good search would include minnimarx - she has fabulous unusual suggestions. I'll try to come back with a link... ETA this link
What is your 10/11 year old reading - nice that this thread rose to the first page, just for me to easily find.
There are many others too.
Charlottemom - as I said earlier, my DS skipped 1st, with a January birthday. Since your son was in multigrade already, how did he do with the older kids? Would some of those same kids be in the 2nd/3rd grade class next year? I agree with BWBShari - many kids will rise (or fall) to the level of the kids around them, and by 2nd grade, most kids are noticeably more mature.
Briefly, our DS's experience was a skip to 2nd in our local school district, which involved switching schools as well because we have grade centers. We made sure that he had playdates with his friends from K when we made the switch. We transferred him mid-year to an out-of-district school for HG kids, the week after he turned 7. I think these 2 moves would have been a lot harder if DS had been older, with more established friends. As it turned out, he made friends pretty quickly the first half of 2nd, and even more quickly when he got to the HG school. He gets to stay with the same group of kids for 3rd (and beyond, if the school works out).
My theory is that the younger the kid is, the less he knows about what to expect, and so the easier it is to make moves. My DS went to 3 different schools in 2 years, with no big troubles. (Yes, little troubles, like bad handwriting and some complaints about gym being too hard.)
