This is only my opinion,
If he's not on an IV and at a clinic from the description you gave of his condition, and if you aren't ready to give up and you don't think he is, see if you can get a sling for him to set up at home. Your vet should know where you can get one to use, if not we can hopefully put out the 911 for one. There are rescues and other orgs that have slings on loan.
Sounds like the quicker you can act the better from what you're describing. I would get gobs of Pedialyte and use it along with colloidal silver as the liquid in a senior feed mash with bran (no corn oil), oats if he'll eat them. No more A/Bs.
More thoughts coming.