@Irish - cool of you to build for your son. Also being a big guy (about 125kg likely fully geared - about 120kg in boxers), let me give you some real world experience as a big guy esk8 rider…
You don’t need, nor do you want a 50+ lb esk8. (no offense i see you have Fiik - just about portability when not riding).
Instead get two 63mm motors - and gear down. I don’t mountain board but do ride fairly often and have had zero issue. a 4wd might be cool too, but not sure how you link them to control them all at the same time - servo splitter and 2 canbus pairs would be my guess w/ VESC.
I would stick to motor/belt setups and not hub motors unless going to 4wd. Even then i don’t think there are any mountainboard hub setups… you want to gear down to have hill climbing ability.
I’d look at the trampa board dual motors - or build one similar if you want a big guy mountain board. Get the new VESC 6 or try the VESC X to get a bit more amp handling. 6374 motors if you can squeeze them for the most power possible. Totally doable.
I’ve ridden dual 50mm, single 50mm, single and dual 63mm.
HTH - GL!