Does he have much experience with computers? Not sure if it’d be easier to set him up with something like ChimeraOS that just runs Steam and the game or a basic desktop environment so he can write stuff and browse the internet.
In the aged care home I work in, we’ve had a few who can work a smart phone but most can barely work their TV. Being into RC planes might work in his favour.
Try flashing it externally. Raxda sells an eMMC to MicroSD adapter for $3 on Aliexpress that seems to work fine for me.