Heyy i had my baby 12 days ago hes currently in...

Heyy i had my baby 12 days ago hes currently in hospital as he was 8 weeks prem. Im expressing every 4 hours day and night and taking it to hospital everyday but im noticing im not producing as much (40-50ml each side) i want to make sure i have enough milk for him i was getting about 70ml each side few days ago seems to have dropped off a bit . What can i do to ensure i produce enough milk for him Any advice apreciated :) Thankyou