I know how you are feeling, I had a dog go off his food and he just wouldn't eat. Saw the vet weekly as I was so worried. But the dog would eat bits of what we were eating. A few crisps, or a little piece of ham. Just tidbits really.
This went on for well over a month and he lost a lot of weight. Yes RSPCA campaign dog describes it well. In his case, he had discovered the delights of cat food and was 'holding out for that' we followed the advice given by Dorwest Herbs below, but used Brewers Yeast tablets, as it was in the days before t'internet.
Have you tried tins of cat food? It may well tempt your little one since it seems more attractive to dogs?
Normally, the advice would be to fast the dog for a day, then start with a small portion of their ration, increasing as their appetite increased. But with such a tiny, underweight dog, it may be the last thing you'd want to do.
Dorwest herbs have the following advice
Loss of Appetite in Cats & Dogs
This is often a sign that a dog or cat is 'off colour' or coming down with an illness, and so should be closely watched for symptoms of any developing illness. However, for normal healthy adult dogs or cats, or ones that are just "poor doers", the following treatment will help.
If possible give a 24 hour fast before starting treatment, preferably with a laxative such as Natural Herb Tablets. Then start by giving a small meal, of around one quarter of the normal amount of food, giving the recommended quantity of Malted Kelp Tablets half an hour before the feed. If food is refused, remove the bowl and do not offer any more until the next meal time - this alone often works like a charm. Although it may seem hard, do not be tempted to give tidbits or attempt hand feeding.
When the first meal has been taken, gradually increase the food at each meal, still giving the Malted Kelp Tablets until, after a period of four weeks, normal sized meals are being eaten eagerly. Malted Kelp Tablets not only improve the appetite but as a result will encourage healthy weight gain and vigour. These uncoated tablets can also be crushed and sprinkled onto the food if it is impossible to give them before the meal.
Don't forget you can contact Dorwest on 01308 897272 for advice on the use of our products for treatments for your dog or cat.
http://www.dorwest.com/Page/Appetite