I don't understand, are his ribs still broken? What do you mean leave that issue behind? Sometimes the ribs can heal on the own, but I would imagine a dog being lethargic would be one of the symptoms.
A three pounder might always have issues with hypoglycemia, (low blood sugar) you should study the condition be familiar with the symptoms. One of the symptoms is loss of appetite, and since food is the thing they need to raise the blood sugar, this symptom can be dangerous. Many people recommend free feeding because of this, and this allows the blood sugar to remain at a normal level at all times. Other symptoms include drowsiness, shivering, collapsing, disorientation, seizures, listlessness, depression, muscle weakness and tremors. Nutri-cal, or white corn syrup can be rubbed on the roof of their mouths for fastest results in getting blood sugar levels back to normal.
Hypoglycemia Symptoms and Treatment
Some tinies are small due to hereditary factors, however others are small because of health problems, I would want to make sure there isn't an underlying health factor such as liver shunt because what you feed your dog needs to be adjusted if this is the case. Glad your little guy found a good home. Hope you vet is familiar with working with tinies and adjust medicines and vacines accordingly, my guess is one shot fits all, and the rabies shot was just too much for the little guy.