Great answers on this thread.

I'll just add a few questions to ponder. No need to post answers, just some things to think about:

1) How does your son feel about circumstances?
About the K - 1st grade split this year?
About his friends in each grade?
About possibly moving to 2nd grade next year?
About possibly being full time in 1st grade next year?
About possibly being in a 1st grade - 2nd grade split next year?
I ask about this because the IAS weighs the child's preferences, knowing the child's level of motivation/satisfaction may help determine ongoing success.

2) Do you read with your child at home?
The IAS considers the level of parental support.

3) Have you read the acceleration roundup of various experiences and viewpoints, including early college?
This collection of threads/posts may help parents become familiar with, anticipate, and plan for common issues of acceleration.