My son is 3 and half months old now and is weighing about 6 kgs. He is not becoming chubby and he lost around 300 gms frm past 15 days( he was 6.4 kgs then). He is not exclusively breast-fed.His buttocks size are the same now like when he was born. am worried if his weight gain is healthy or if I need to consult a doctor. Please advise.