i mean, why have gears and a brake lever when you can have a coaster brake? or even a kickback hub with coaster brake? he could use a pretty "normal" bike.
lack of an opposable thumb doesn't seem like that big a deal. a foot forward bike would definitely help with stability, but i think you could also get a bar setup that would work well enough - i'm trying to imagine what would be easiest to grip AND push forward against without a thumb, and a few things come to mind. it seems like what might actually be ideal is some very upright flat bars or maybe even bullhorns or bar extenders - that way he can bend his wrist to be able to push forward with his palm while still wrapping his fingers around most of the way.