My son is 3.5 months old and is around 6.3 kg (birth weight- 2.7 kg). He is breastfed and formula fed. He takes around 60 ml formula every 2-3 hours and in between He takes breast milk as well. But at times this gap increases and generally at night he drinks in 3-4 hours around 90 ml of formula. Is it normal or he's underfed? I have tried increasing the amount but he rejects the additional amount.