Right, in front of a computer.
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roll bar, lift and bigger wheels/tyres, but not sure what is achievable as yet.
You have an SJ80, which has a number of key differences compared to the more common leaf sprung car.
I'll second Zook_Fan's comments in relation to a "roll bar" There are many cosmetic bars around which might actually be worse in a rollover than nothing at all, especially as your son is very tall - it will put his head back so the cross bar will be quite close to his head and the vertical bars might be worryingly close as well.
There's really only one commercially available "lift kit" - it's a 40mm kit which uses ultima shocks and king springs. I think it's OK but not awesome. Basically, the coil sierra suspension design is allergic to lift.
Tyre size increases need to be limited. 215's are 28" tall and won't interfere with the gearing too much and fit easily (on standard wheels) going beyond this starts to affect gearing negatively and other fitment issues start to rear their head.
Correcting the gearing is a bit more complicated than for a leaf sprung car as you can't re-gear the existing transfer case, you'll need to swap in a transfer case from a leaf sprung model along with the jackshaft and front driveshaft.
As for moving the seat rearwards, this is a bit of a compromise. Whilst I'm pretty short, we've had some very tall club members with Sierras. (up to 6'7") Moving the seat back might help, although you need to be careful to ensure the floor or rails don't crack, but interestingly, moving the steering wheel closer to the driver helps a lot, because it's all about freeing up knee room - you want your knees between the wheel and the dash, not trying to share space with the steering wheel. I don't think trying to get the seat back far enough to get your son's knees away from wheel is feasible - the ergonomics would go all wonky.