18 yr old cat pooing and weeing indoors

[DropInTheOcean54]

My cat has always toiletted outdoors - nips out through the catflap and mooches about in the garden. Recently he has taken to pooing and weeing indoors - usually in the kitchen or hallway. He has been to the vets for a urine infection a couple of months ago and was treated for this. I don't know what to do? Get a litter tray? Take him back to the vets? is he nearing the end of his life? He is very skinny now but has a good appetite and loves his evening cuddles on the sofa with a nice warm lap to sleep in.

Sadly this is what our ancient cat started doing at the end of his life, and he became very thin. He didn't appear distressed by it and died in the garden one day.

Get a tray it’s freezing out there & he’s 88 in human years.

greta my cat is really skinny too. I think it hurts him to sit down as he really takes his time to lower himself down. I hope Charlie just passes away peacefully in the garden too. WE just don't know what to do as humans....
Fluffy I have an old litter tray I think so I will dig that out and get some litter for it today. He does go outside by choice too and it is quite mild here but it is a consideration.