How many ML formula for 3 weeks old

#firstbaby #advicepls Hi! I read up on google that my 2 going 3 weeks old in 3 days time should be drinking 120ml per feed however when I gradually increased to 90ml at 2 weeks, he will only drink 60ml. Every feed I’ll do 90ml but he will always drink exactly at 60ml every 2 hours up to this date. What should I do? I’m afraid when he’ll go for his 1 month check up, the doctor will say that I’m underfeeding him. Kindly help as I’m a first time mum!