If he's been in terrible conditions like that for eight years and is still alive that must mean he was 'maintaining'-- certainly not a healthy maintenance, but if he isn't in the middle of a crash and burn (like suddenly becoming non-responsive or something) that makes me think he may have a decent chance of pulling through with proper care. I try to be optimistic with rescues because I think giving things your best shot as long as the animal still has some fight left gives you the best result. He's not out of the woods by any means, but I think there's a decent chance you got him in time to make a difference.
At any rate, good luck with him and keep us updated!