Ensure that he sucks a little before you feed him expressed milk . Even though he may not have effective milk intake through sucking . This is called non nutritive sucking . This will ensure that the baby develops sucking and doesn't get used to just having milk fed to him passively . Within a few days as he grows in weight and strength he will start sucking efficiently.  Don't worry about that .     I don't think that milk output in your case is deficient as your other baby is thriving on breast milk alone . Expressed breast milk is never as effectively extracted as a baby can do with a strong suck     But still if you want to take medication , we generally prescribe Tab Perinorm 1 tab thrice daily for 10 days       Sufficiency of milk will be guaged by his urine output , if he is satisfied for 2 hours or so after a feed or is consistently wanting more and weight gain ( about 150 gms per week or 20 to 30 gma per day )  If he is finishing the entire 40 ml in one go , maybe he needs more . In that case you could add 15 ml or so of Nan pro over and above the expressed breast milk ( EBM) Try not to replace a full breast feed with Nan pro . Instead give Nan pro over and above the EBM to cover the short fall