If he really is as learning disabled as your examples show, I’m not surprised he has struggled to do your re-stocking.
I’m no pro, but I’ve done a little bit of work with folks like this and it’s very hard to really get into their head enough to understand & accept just how limited they are. And also to understand how uneven their limitations can be. They’ll have small bright spots in the dimness that’ll amaze you.
In his mind you’ve asked him to learn a thousand tasks. Find the Canadian Club 1.5 Liter size. Find the Canadian Club 1.0 Liter size …
Normal humans take for granted the ability to find the common elements in tasks & can reduce all of restocking to “Notice what’s missing on the shelf. Go to the corresponding place in the warehouse & get more. Bring it back.”
He (probably) can’t make that generalization any more than you can do tensor calculus in your head.
Perhaps you could focus on one small area. e.g. The front half of the first aisle out front has all our vodka. In back, that’s all within a 6 foot span of shelving on the left. Make him the vodka king for a month & have somebody else do the rest of the store. Yes, a month.
IF that succeeds, add another line. If the second line befuddles him & he can’t even do vodka any more, go back to vodka only, start training from scratch (it’ll go quicker this time), & accept that’s about as far as you’re gonna take this guy.
It can be frustrating as hell for you and for him. Between the boss, you and him you may end up with a simple charity case, where he does what “work” he can to feel good about himself, while you & the others do the real work (some of which is undoing what he did), and the boss pays for it all so he can feel good about himself. And that’s not an altogether bad arrangement.