We're all thinking on the same track; as with PG, I would suggest the C111 for a more relaxed and progressive jibe style (ST is a bit more technical).
With 75kg, his ankles will be damp but knees should be dry
Any bigger volume would be too much overlap to FT138 range, and not so sweet or reactive into the jibes. Tacks are possible - but reasonably challenging for most intermediate sailors - on all boards around these dimensions. Hopefully having a sweet carving ride will reduce any reliance on tacks and soon have him slingshotting in + out of the turns.
Cheers ~ Ian