If you got this from a bat chart, then it's wrong information. Most 9 year olds are swinging a 26/16 or a 27/17. Only the biggest kids have bigger than that and it's arguable that they shouldn't either.
Only you know if your kid can truly swing the bat. But it should not look like they are swinging a log, they should be able to whip it through very easily. Too heavy of a bat is the single biggest detriment I see in youth hitters.
Brand don't really matter. I'd probably go 2 5/8 diameter for a smaller kid. Don't spend more than $150 (max end if you want to go expensive).
It’s always a qualified depends - 27 or 28 likely - would stick with drop 10 - go lighter it’ll ring your boy up more and then he’s not going to want to swing hard to avoid his hands hurting
However -
My just turned 10yr old swings a 29 drop 8 just fine (he uses that or his 28 drop 10) and he’s just 4’3” and 62lbs - typically depends on velo he’s facing - he saw 50-59 this weekend (10u) but it’ll bump up into mid to upper 60s on average when he’s playing 11u
This is what we’re working toward though as a baseline
10 - drop 10
11 - drop 8
12 - drop 5
13 - drop5/drop3 (school ball is drop 3)
14 - drop 3
The idea of bats right now make me sick - we’ve broken 4 (2 wood, 1 composite, 1 allloy) recently