infant malnutrition is one of the most common cause of them being underweight so increase his diet.. aftr each meal give him syp pepsizyme 2 tbs..fr 15 days syp albendazole at night the whole bottle this is for worms in his tummy which might causing malabsorption syp levocetrizine 2.5 ml at night fr 3 days tab limcee half tab daily to boost his immunity if he cn take it by himself fine or just dissolve it in lil water n make him drink syp dexorange one tbs thrice a day fr 15 days..it vl take care of anaemia