That makes sense and was good of polarbear to read closely enough to see that you'd have two 9 y/os if that wasn't a typo.
I have a couple thoughts.
1) It sounds like he is still on the older end for grade depending on where your cut for K entry falls. For instance, where I live, kids have to turn 5 by as late as October 1 to start K (each district has its own cut off date and many are earlier than that). Where I grew up, the cut was 5 by December 31. Granted, there is a ton of redshirting, but I'm guessing that he missed the cut off for starting K by a few months.
Skipping a grade shouldn't make him drastically younger than the other kids unless, like my area, redshirting is terribly common. Our youngest, who made the K cut by three days, is as much as 18-19 months younger than some kids in her grade due to redshirting of other kids. I'd strongly consider a grade skip.
2) With an older sibling who is, presumably, only one grade ahead you may run into some resistance on a grade skip, however. I haven't seen the most recent version of the Iowa Acceleration Scale, but when it was being used for my oldest to consider a grade skip having a sibling in the same grade into which the child would be skipping was an immediate disqualifier for consideration. The other things that ruled out considering a grade skip were an IQ that didn't at least hit around 130 (or 115 if the kid just missed the cut date for K by days), or the child being unwilling to skip.
One of the board members who no longer posts here frequently if at all did have a child who skipped into the same grade as a sibling successfully, though. Just be aware that you'll probably have a more uphill battle on your hands with the school especially if they take the IAS literally on every point. How do you think your kids would handle being in the same grade?
I do know that it has increased self esteem issues for my youngest to have her older sister be three grades ahead of her in school when they are almost exactly two years apart, but I couldn't do wrong by my oldest just to assuage her sister's feelings and keeping my oldest in the grade she was in really wasn't working.
