This would make me suspicious of a health issue as he is so senior a dog for his breed.
It could be a sign of many health issues including liver, kidney, heart or digestive tract problems.
With his age I'd suggest planning an in person vet exam for him to find out what is going on.
This breed can suffer from bloat and you can read the symptoms here
But the fact he has a picky appetite may mean he is feeling unwell from another health issue.
If your dog might have gotten into anything toxic, appears dehydrated (see if a pinch of skin over the shoulder blade stays stuck in a tent form or flows back down normally when released), has a rectal temp over 103.5F (normal is 100.5-102.5F), has gums or tongue that look white, blue, gray or yellow tinted where they are normally pink, has a slow color refill if you press on a pink area of gum or tongue, or is becoming lethargic and non responsive to you then you may want to get the dog in to see a vet today.
Hope this helps you!