Not for the easily nauseated

So our oldest cat, the one we took in as a stray four years ago and who has to be 12-14 years old at this point, just did a big poo on the carpet in front of the sofa on which I was sat reading... I looked up to see what he was scratching about and there were these turds lying there and him trying to dig a hole in the floor to bury them.

Does anybody have any advice on what to do with a cat who's used to doing his business outside but either can't or can't be bothered anymore to do so? Getting a littertray in is difficult in our teeny tiny postage stamp of an appartment.