I totally understand your frustration.

Other options, if you care to try, are:

An EMPTY litter box.
Newspaper, as suggested.
Puppy wee-wee pads at the bottom of the litter box.

And considering I am REALLY against outdoor ONLY kitties, even I think that having him outside would be better than putting him to sleep. As long as you feel he is still comfortable and loved. Do you have a garage or shed he could bunk in (maybe via a cat door).