Most important information needed here is, whether he is being breast fed or formula fed or being given cow milk, goat milk? If breast feeding is going on, then do not worry about weight gain. He will develop according to his "prakruti" or "constitution".
If he is being fed external milk, then he may not be digesting the milk fully. If that is the case, boil milk with equal water added till water part evaporates. add a small piece of fresh ginger root or 1/4 tsp of ginger powder while boiling starts. This process makes milk less allergic to children, if there is a lactose intolerance or casein intolerance.
If you are feeding external milk, just add a powder of one dry date fruit before boiling. Milk+dates is a weight gain diet when children are being externally fed. But in this case giving some bitter herbs to take care of colic, worms possibility is also must.
answered by S B