Feed him special fresh, not canned food, only when he is awake and when you want him to stay awake while his food is metabolized by his tiny body. If he is not throwing up or having loose movements, he may have a systemic yeast infection, or did indeed pick up something from another dog.
However, if he's still eating, it's a good sign. Make certain he has plenty of clean water as well, and is not too cold. The floor of any room is the clodest area. Also shredded pine shavings in bedding can make an animal very ill. Is he
urinating normally, and does it smell particularly strong?
Acidophylus for pets/children is available in refrigerated section of Whole Foods. It is not a medicine but replaces anything good that might have been harmed within his bowels and mucous membranes. Also, give him no dry cereal or dry foods until he is well. I'd feed him only organic: fish, chicken, as in baby food meats for a time, mixing in a raw slightly beaten egg, some finely chopped broccoli or frozen mixed veggies, with a small amount of reg. white bread crumbs, and sprinkle 1/2 capsule of Acidophylus and mix well. It tastes like milk, and pets don't seem to notice it.
Also always sprinkle food with garlic powder to help make his pest resistance stronger. Use no sprays, chemicals, or bleaches near him or in the same room he is in or might come into. He is much smaller than a human so what might not affect you may be killing him. Be very cautious, alert to every thing you are doing. He may need more petting during any sickness, and may feel you abandoned/ betrayed him to the vet who may have hurt him a bit during an exam? He should begin to respond with TLC and better food.
'No more canned of any brand', is my pet motto, my having lost my two precious young best dogs to what is inside canned dog food: Chopped up diseased animals, according to the officials! I rely heavily on tuna in water, adding my own "small animal vitamins" from the pet store which are not expensive at all. Once he is well, you can then add a very small percentage of dry food. Because he is still young, he may still be teething and need a tiny rubbery toy
to chew on that fits his tiny mouth and makes no noise.
He may miss you if you work, so you could leave the TV on low for him while you are gone on a children's program so he can hear the chatter and laughter of children all dogs love. He may benefit from having a stuffed dog as a "friend"
as well, and should you have another friend with a small dog you could make a play-date for them to romp together? All creatures benefit from companions of their own kind, size whenever possible. If he is weak, crawling near the floor when he walks, that is usually some sort of abusive behavior. Stay alert to all visitors and others who play or hold him who may be sadistically harming him without your knowledge. You are all he has, remember. Wash his bedding
frequently but dry in the sun, if possible.
Also, the added fresher more natural vitamins should help if he is depressed. (Dry foods can make their skin dry and itchy, as well as over-sensitive to petting/ touching. Keep your voice calming and quiet. Is he house-broken by now? If not are you gentle with your scolding or loud and angry? Do you shampoo him, take him to the groomers? All of these things need to be considered which I think you are trying to do. God bless and help you and your dog. : )
God bless and help you and your pet. : )