Require suggestion to gain weight for my baby

Patient: Hi My son is a preterm born at 32 weeks with very less weight (1 kg) and till now he has not gained much . Now he is 13 months old and weighing only 6.5 kg. activity wise he is doing great much physically lagging behind . He was drinking milk before a week now he is refusing completely . Can you suggest me any alternate or any additional supplement can be given to him ..Please do the need ful

Symptoms: Throat infection , coughing , slight cold and had vaccination recent (Typhoid and haverix)