Considering all this guy has been through, it's no wonder he wants
nothing to do with eliminating outdoors. He probably associates that
with pain or trauma. Please keep him indoors and let him use the
litterbox inside. Do you have a closet or area where you can put the
box? I lived in a small apartment and devoted one closet to the box.
As for the smell, if you scoop or change the box often there really
shouldn't be much of a smell. I am not familiar with Natty Cat litter,
but am guessing it's not scoopable. You can still scoop the solids
daily and keep odors to a minimum. You can also switch to another
litter that might control odors better. If you do this, switch
gradually or he might get upset and not use the box at all! Also, make
sure the box has low sides he can get into easily. I'm sure he isn't
as mobile as he used to be!

I'm sure this guy has had a rough time. Please don't force him to do
anything he's not comfortable with. I hope he makes a full recovery.