Hi Gadge, at this stage we have F-N-R selection on the gearbox but my concern is if he is on the sloping ground and he puts it into neutral the thing might become a billy cart before he can get it back into gear. It also may not release from the gear due to the load on the gears so forward/reverse switching could be a good idea.
It's both easy and cheap to set up; the most work is making up the mounting panel for the switch and relays. Or they can be set up in a small diecast aluminium box, which can be weatherproofed quite easily.
30A rated 12V SPST automotive relays are cheap, and the switch is ~$5.
For example: Switch -
https://www.jaycar.com.au/dpdt-6a-240vac-heavy-duty-centre-off-standard-toggle-switch/p/ST0576Relays - https://www.jaycar.com.au/products/electromechanical-components/c/22?q=%3Arelevance%3Acategory%3A221%3Acategory%3A221E&text=