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It may be he needs to have his teeth cleaned. If he will let you check out his teeth to see if they are covered with tartar in the rear ones.
Often cats with dental issues will click their teeth and stop eating their hard food preferring the canned food which does not caused pressure on their teeth to eat it.
You can read more about dental issues in cats here
Its not uncommon for a cat with dental pain to be acting differently than it usually would.
Hope this helps you!
I applied Frontline plus on my mail 10 year old cat