Hello and welcome - My name is ***** ***** I am an experienced small animal veterinary surgeon, I will be very pleased to work with you today and will try my best to answer your question to your satisfaction.
When a cat starts to eat odd non food items like cat litter this is termed " pica " . It is not something you would expect to see in a healthy animal and is most likely occurring in your case because his health problems especially the kidney disease have started to affect his brain. Brain involvement such as mild dementia are also sometimes age related.
Probably there is not much you can do to help Titch other than concentrating on managing his present health problems but of course these are chronic.
Eating small amounts of cat litter may not cause too much of a problem but eating more than that could cause alimentary tract impaction which would be a very serious situation. You should do what you can to prevent him doing this, perhaps a change in cat litter to another sort entirely or consider using soil from the garden or peat from the garden center.
I hope I have covered your question fully enough but if you would like further clarification or to talk things over a bit more then I will be on-line for the next hour or so and I will be more than pleased to continue working with you.Regards,Scott