Suggestions for a senior dog who now has a finnicky appetite?

First have a vet check to make sure there is no health issue such as painful joints or teeth/gum issues..... those aside, dogs will become picky when the owner runs around to find something better/more tasty and an older dog soon learns to be picky with food knowing if it is you will find something better and better still the next time....

If you are feeding a good quality diet then you should not be concerned, but stick to meal times and not free feeding.

I feed raw/bone and find my senior dogs will eat the same as long as their raw is chopped up small, I provide additional vits.mineral.trace elements and increase their neem powder which keeps their immune system at optimum levels....... and I also have high IU Vitamin E in ( just in case) which makes them hungry ( which is what vets give to encourage a dog to eat)

From google search using just your question.

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If you have done any research you've already seen these sites.

I agree with everybody else - who suggested a vet check.  Senior dogs should have one, EVERY SIX MONTHS rather than once a year, as they can go downhill, quickly.  It is important for your vet to rule out common causes for a lower appetite: sore gums &/or cracked & infected teeth, a deep ear infection - which can cause pain when chewing, joint pain or CANCER.  A full blood panel (including calcium levels -which can be raised due to certain TYPES of cancer) plus a urine check, can be very informative for the vet.

Raw (with ground bones), pre-made raw, or soft (easier to digest) diets will be MUCH tastier, and may also be helpful for a senior dog, esp if this one has very worn down teeth.

Senior dog = not many days left in this world

If you were on your last years what would you eat I can gaurntee it wouldnt be some grain free wet slop bullshit get him a roasted chicken once a week and throw in some peanutbutter from time to time maybe some cheese. Feed your dog what he actually wants to eat what ever that may be. My dog lived on raw meat for his last year absolutely loved every bite went from a dog who wouldn't eat to begging for more

If your dog hasn't had a senior wellness check you should have it done, the vet will do a full blood panel and can rule out or identify issues.  If healthy sometimes a change in kibble will help, small pieces are generally easier to eat.  You can add a topper if it helps.  I add water to my dogs dry food because in their stomach it soaks up a lot of liquid,  you can add a few tablespoons of plain fat free yogurt, plain canned pumpkin,  unsweetened applesauce.  Avoid adding anything fatty as it can cause pancreatitis in some dogs.

Senior dogs often have a slow kind of appetite,

So just give your dog small amounts but often my friend, I hope it works out. 

Put him down.