Nice to hear from you.
So, has your son sailed without a centerboard/center fin/daggerboard yet?
Having him learn to "rail" a board with no center fin is going to be the trick.
The GO 170 planes very early for light weights, so if you have enough wind, and
can get him out on the GO I think he will like it better and plane sooner.
A smaller GO 140 would seem even better, but they are pretty rare.
One of the older Pro kids formula boards might also be something to consider
if you can find one.
I just wish you had access to a 3.3 m2 Retro Ripper.... probably significantly more low end
power than the Super Freak due to more designed in draft!