Be careful where you make your compromises.
With cranks too long you’re potentially messing with his muscles and knee joints and he’s not actually gaining any advantage.. Only weakening his knees.
Whilst not necessarily pimp, you can pick up Sinz bmx cranks in any length from 120mm – 175mm. You might find that at 10% height for crank length he should actually be closer to a 130mm crank.
There plenty of bike information available that references knee problems with long cranks, and that longer length doesn’t always equate to more leverage/power transfer.
I’d suggest his road/cross cranks are too long also.
Careful.. you’re nurturing a talent.. don’t break him just yet 😉