I suppose it depends on how he likes the fit of that kit, as to whether you go a size up again, then.I got him 12/13 in Barca kit and it’s not exactly big on him.
I honestly don't know how most places size kids's stuff. Must be basing it on average sizes from abar 100 years ago because my lads have always been in at least a size up from their actual age. Nightmare.