Hi Jenny
Too right its not nice having a cat who is soiling indoors but there's lots of reasons why an older cat might start to do this. 1.He may be starting to slow down due to suffering from arthritis or have some other illness slowing him down, in which case he is less keen to go outdoors. He may have 1. but also 2. be being bullied by an outdoor cat (or even one of your own cats) which makes him less confident and reluctant to venture outside to poo. So he takes the easy route and does it indoors.
Whilst you are observing what happens in the cat interactions with yours and other local cats I suggest give him a litter tray and put it in a location which is quiet and away from food and water bowls belonging to him and your other cats. How many cats do you have? they may also start to use the tray which means he is less keen to share that. It gets complicated the bigger the number of cats you have. Trays can sometimes need to be provided at 1 per cat plus 1 extra. Cleaning trays is better than him soiling indoors though! Don't forget he might be ill so your vet may be needed to help diagnose and help sort out the problem.
Cathy Wickenden BVet Med MRCVS
Barton Lodge Veterinary Centre
1 Midland Road, Hemel Hempstead, Hertfordshire, HP2 5BH
Tel: 01442 216048 24 hours
[
www.bartonlodgevets.co.uk]