My 18 month Dobi will not eat properly. Sometimes he will eat his dinner at night and other times he will not. I have had him tested at the vet's for all sorts of things and all the tests come back normal. I have tried him an all various types of diets and nothing works. He has gone four days without any food at one time. Other times he will eat only one cup of food a day, other times he will not eat at all. I have had his teeth looked at as well as his tonscils, all is OK there. He is under weight but still loves his training and going for walks. He seems happy and content... I can't understand this. Can someone please suggest something that may help me with my dog.

There may well be nothing wrong with him at all.

I have a 27 month old dog here who is still extremely fussy when it comes to food. Some weeks he will pig-out for the whole week and act as if he has never been fed before, but then the next week he will hardly eat anything.
He is very healthy... just fussy with his food and a little underweight.

My mother had a dog who was exactly the same and once she hit 3 years of age she just suddenly started eating properly and balooned in weight (she is now 10 years old).

You can try some higher calorie diets, they will not only add weight, but usually taste better to the dog as well. Things like adding sardines in oil are high in calorie due to the oil, plus they are good for the skin and coat.
Adding a teaspoon of peanut butter to the meals and mixing it through is high calorie and tasty.

Thank you for your advice. I have tried sardines,herring and a high calorie diet. The only problem is he just will not eat. He is very thin and I am very worried. Do you know anything about nutrigel? It was suggested to me to try this product. I just can't get him to eat. I am worried about what affects this problem will have on his long term health.

Has your vet prescribed any foods? There is a diet out by Science Diet called A/D... Most call it the "anorexic diet"... It is supposibly very palatable. It can also turn into a slurry for you to syringe feed with ease.

If offering him food is failing, you might have to force feed. You might have to put some dog food in a blender with water and mash it up and stick a syringe down his throat.

Have you had his blood sugar tested? How about a parvo test? Any type of blood test done? Its never normal when a dog refuses to eat. He is sick and you need to find out why.

Nutrigel is like a vitamin. If he isn't eating, than it will really not do you any good. He needs food, not vitamins.

You can get appetite stimulants from your vet if you feel the problem is becoming dangerous. Definitely worth looking into.

Nutrigel as Lucky said has a ton of vitamins and can give them a boost and encourage them to eat, when their vitamin and sugar levels drop they feel "off" which in turn can make them not want to eat, which makes their levels drop even lower and the cycle just keeps going.

Do you have a photo you could post? It would be helpful for me to compare your Dobie to my current underweight dog.

How long has he not been eating properly? How often are you changing his food? Changing it too often can result in a very unhappy Dobie tummy.

Just a few thoughts:

Does he beg for yours? If he is more interested in what is on your plate than what is in his bowl, you might try pretending to eat his food and see that gets him interested in eating.

Does he work for 'treats'? If so, how many does he get during the day? Could they be acting like an appetite suppressant? My Dobie girl will work for her food and ignore her bowl if you let her.

Have you tried different methods of feeding him? Or different bowls/dishes? What happens if you put his food in a treat ball?