Cats can be dehydrated with no symptoms. Gandalf was very dehydrated, yet I couldn't tell it, even by pulling up on his skin. He's been treated for 2 weeks now with subcutaneous fluids and is doing much better.

Cats are naturally desert dwellers and drink very little water. You are feeding an animal which doesn't drink alot DRY food. No wonder one of the top diseases in elderly cats is kidney failure.

You can try a change in food, but if it goes on much longer, he is going to get sicker.