I think I'd stay with the board that he's using now for a little longer, or buy the smaller
In either case, 12 year olds (he'll be 12 pretty soon if he's 11 now) grow quickly, and and you will need the larger PK Flare within a year or so.
I can only tell you what I know from my experiences with the young freestyle superstars from Bonaire. Kiri, Bjorn and the others were able to do wonders with larger boards before the PK series was launched, so I'm pretty sure your little brother can make some good progress.
But, put him on a board that's sized right for him now, and he'll progress even faster.
Hope this helps,