I loved radical acceleration. I was accelerated 3 years both times in two different districts - from 1st into 4th and from 7th to 10th. If its a good school and he is treated like any other kid, he will thrive. The first time, I felt really small and recall crying a bit, but was fine the next day. The second time I was just glad to be challenged.
I also had a smaller acceleration in between and for that I was moved into one class for reading and then another for math and then I got to pick which class I stayed with and they moved me at the middle of the year.
So, wht you might do is have him go up to 4th grade (??) for reading period to see if he likes it then add in the other subjects until he is fully integrated.
Writing was a big issue for me - I have never had to take notes before - but that was worked through. My much older classmates took me for what I was.
If he is not accelerated, then he will learn to sneak books into class and pass the time teaching himself.
Even when I was accelerated, I did this anyway. Be prepared for it. Just because its more advanced material, he may "know" it in a minute then be bored again - I was not truly challenged until I got into AP-level classes.
