I would want to actually see him eating the litter before going too far into diagnosis mode...'might be due to his "nosing around" in there!
And, if you have a corn litter product, he might have been "munching"!
All that said, if you check the Symptoms and Treatments page on Helen's site
, you'll see that eating (clay) litter can also be a sign of a calcium imbalance.
Wasn't there a question about his calcium levels (earlier this year/late last year) when you were posting to the CRF list? I think you were using lactulose for his constipation and when the calcium level issue arose, I gave you this reference from Helen's site:
"Some people have found that using lactulose to control constipation has led to hypercalcaemia
in their cats. You may therefore wish to avoid this treatment if your cat already has hypercalcaemia, and consider alternative treatments if your cat has developed hypercalcaemia since beginning lactulose."
My suggestion was that you speak to the Vet about using an alternate, maybe Miralax, if my memory is accurate...
Are you still using lactulose?
Was his calcium level high on the last bloodwork?