I urge you to never let this cat outside again (with the obvious implication
that he will never go "home" again).
His previous people are totally irresponsible or terribly misinformed about cat
care.
His health depends on him staying inside. Even in one room, he is better off
than in the previous conditions that resulted in his current state.
I have rescued "strays" and decided that if anyone ever approached me I could
give them the vet bill and tell them they can have the cat back after they pay
the vet bill. Anyone who cared this little or who knows this little about cat
care is highly unlikely to want their kitty back with this condition. No one
has come looking.
In one case I went to the owner's home in another city (the cat had a
mirco-chip and lived in a different city than where I found him in bad shape ~
dislocated jaw and broken leg) and left the cat at my home. I did not contact
the man again and did not give him the cat back. I believe these cats cross our
paths for a reason. I know some might not agree but this is the path I have
chosen.
Laurie