If I had a dog that I knew was healthy but absolutely would not eat and was too thin for my comfort, there are 3 things I would do. He is a big dog around fifty pounds or so he is not even a year old yet.
1. Make some Satin Balls and give him a few (how big is he? How much should he weigh?) for each meal.
2 lb Hamburger (High fat, raw)
2 cup of his kibble
2 oz Wheat Germ
2 oz Molasses (not syrup)
Mix together and roll into 1 inch balls. Refrigerate of freeze. Thaw as needed.
2. After a few days to a week of the Satin Balls for meals, after he has gained weight, I would get a high quality canned dog food. Offer a little mixture of 1/4 kibble mixed with 3/4 canned dog food at meal time. **Leave it in the bowl for 15 minutes. If the dog doesn't eat, pick it up and cover it. Put it in the frig till next mealtime. Keep doing this until he eats it.
He can afford to get hungry now, so let him. If he's healthy, he won't let himself starve when there's perfectly good food in front of him. After a few days on this mix, mix in more kibble. So now his meal is 1/2 canned and 1/2 kibble. After a few days, up the kibble ratio to 3/4. Keep doing this until he's eating full kibble.
If at any time, he refuses his food, go back to the **
in the step above with the current mixture.
3. No more people food! No treats. None. Until he's eating regularly again and eating only kibble, he gets NO other food.
Well... 4 things. Don't worry over him. Be very disciplined about this. It's for his own good.