Aw, poor old fella!
(Bonkers, I mean. Not you, Ragman. :wink: )
I have been in this sad position many a time. (My oldest cat ever reached the grand old age of 23!) You're doing all the right things. He's lucky to have such a caring human looking after him.
When mine have reached this stage I have simply indulged them (within reason of course, nothing that would make their condition worse). So, if Rats fancied roast chicken, then roast chicken it would be! If he wanted a bit more then he got it. A spell outside in the sunshine in a sheltered spot? OK, outside we'd go! But I'm sure you're thinking's pretty much along the same lines, yes? A little pleasure goes a long way.
Good luck & keep us posted.