Shih Tzu Is a Picky Eater?

These are really adorable dogs and I know you are concerned about his health but your "worrying" may be part of the problem. First question would be has he been to the vet to rule any health problems? Second - you mention "dog sitter" so I assume you are gone during the day and someone (your home or theirs) is taking care of him in your absence. Do you know if he is getting a good amount of exercise as usually a dog will have a better appetite if they get regular daily exercise? Some dogs (even small ones) need a lot of exercise so be sure of this before going any further. He may not be hungry because he isn't burning off enough calories.

Most breeders say to make a choice as to the food you want to feed him. Don't add anything to it or if you do, just do it occasionally. Offer him the food. If he doesn't eat it in 5 minutes, remove it for a few hours (or when he appears hungry). He'll eat when he is hungry enough. I would not start adding other treats (canned food, people food) or changing foods, unless you change it to canned food for good.
I do not know if you ever place him in a cage/kennel in your home but here is a suggestion: Put some dry food in his kennel so when you put him to bed for the night he may eat it while you are asleep. (not very much to start). If you do not use a kennel then do you have a small room that could be used but I would not upset with too much change.
The only other suggestion that I have is pick out a good food product, stick with it and maybe occasionally add some cottage cheese just a little - youll soon know if he will eat it - just leave it for 5 minutes and remove it - be patient and do not act like there is anything wrong because he is not eating. Remember - exercise is vital to his health.

Here is a link to the American Shih Tzu Club about picky eaters.

americanshihtzuclub.org/fussy_eaters

I don't feed my dogs canned dog food at all. None of them will eat it. Therefore, I am forced to visit the store and buy the packages of scrap meats they sell for animals.

I boil the meat and cut it into bite size pieces. Afterwards, I add cooked pasta or rice to the meat. They all ove this and it is very healthy for them.