Not gaining weight


My son is 4 months old today. From 2 weeks he is just taking 400 ml formula in 24 hours . He is having 3 wet nappies only. Every 3 hourly he just take 50 ml. I try to offer every 2 hourly but he refused. After 3 hours he just take 50 ml of formula. I went to paediatric but he did blood test and results come normal. I tried everything but no help. He is not even gaining weight. I don’t have much supply so I give him more formula but he just doesn’t want to take. He sleeps more than usual. Other than he is very active. He poops once in a 2 days. He always does green poos.I ask doctor is he been dehydrated but he said no.

I am worried so much. What should I do? Please help.