Aw, poor lad. I hope he doesn't have stasis.
Jane, I know you do everything to keep his surfaces as soft as possible and I don't recall his age, don't think he's old enough for back problems, but does he have an odd gait or stance that could've contributed to his hocks wearing down like that? I know Fiver had a bent way of standing for many years but can't remember what came first: that or his hocks wearing thin. Bunnies' feet sometimes begin to bring to mind how car tires wear unevenly in certain spots due to how they're balanced or set. It's a goofy analogy but I think rexes seems to manifest early signs of back trouble through having worn or furless areas on their feet.
I know rexes are prone to this condition due to the nature of their pelt, so I'm most likely talking out of my hat but I'm as flummoxed as you are as to how to address it. I take it even wee doggy socks won't work because he simply wants nothing on his feet.
Could Manuka honey have an application here? Or would that be bad for him to lick it off?