Poor dude is some pretty rough shape. I'm glad you're gonna try to rescue him, He definitely will need a LOT of care and vet visits.
From what I can tell what's wrong with him from that photo,
He has a seriously
BAD Case of gout in his foot (see circled photo) that is probably really painful for him. I know Drache613 (Tracie) one of the mods here sells Gout ease at her website Bug-De-Lite.com, that could help him a bit.
View media item 61044 That's my main concern.
The Spots could be Lesions or hints of a fungal infection (I doubt it though), Or something internal, It's hard to tell.
View media item 60607He also looks very Dehydrated.
The way he's lying also is concerning but I'm not too sure about that.
Whatever is going on with him, I wish you great luck on rehabilitating him.