If he has been vetted and there is nothing physically wrong, his loss of appetite may be his way of 'signing off'. Humans and many animals lose their appetite and just don't feel hungry or want to eat. Hence his only eating really special yummy things, not routine foods. 16 1/2 is a very nice old age and all his time is Bonus Time with you.
We recently lost an elderly human friend who pretty much starved to death. He stopped eating, and eventually would only take tiny sips of water. He was never hungry or thirsty, we had to really encourage him to have a sip. If he had been a pet, I would have taken him to the vet for euthanasia, as he got so thin he was in pain just lying in a very soft bed. But we cannot be that kind to our human friends, only to our beloved pets.
Keep giving him special yummies and encouraging as much activity as he will accept, plenty of fresh water and a soft place to sleep. Talk to your vet, he may give you something g to stimulate his appetite, and he will be able to counsel you about signs that 'it's time'. It is a very hard decision, especially when it's just old age.