What food did your vet give you?
When my cat refused to eat, I fed her chicken broth baby food through a syringe or off my finger if she had the energy. I also grinded her dry food into powder and mixed it with mashed wet food and warm water.
You can also try homecooking and blend/puree all the ingredients into liquid.
Here a recipe from Ann. Martin's book (for active dogs so it would put on wieght):
1 pound ground meat fat included
4 hard boiled eggs
1 cup cooked oatmeal
1 cooked potato
1/4 cup wheat germ
1/2 cup raw carrots
1/2 cup raw green vegetables
1 tbsp olive oil
2 tbsp minced garlic
I'm not a fan of multi-vitamins but I would give vit C. Also, instead of olive oil, I'd use fish based oil. I'd also add rosemary and a tiny amounts of kelp. If this is something that works, variety is key - you'd need to use different protein, veggie and carb sources for a balanced diet.
This site has a couple of recipes that are "to put weight on" as well: