Hi Val, sending hugs to you and your precious elderly friend.
I had to face that situation with my first Yorkie, LacieBug, not because of old age, but because of terminal illness. Every time I took her to the vet for checkups and more meds, I would ask Doc, "Is it time?" and Doc would always reply -- Cindy, listen to LacieBug, she will tell you when it is time." So, my answer to you is --Take him to the vet for a good checkup. Make sure that he is in no pain and see if there is anything medically you can to to make the remainder of his life more pleasant. Then take "listen" to what your friend tells you -- he will tell you when it is time to let go and let him fly with the angels.
Hugs and more hugs
