My baby is three weeks old, weighs nearly 11 pounds and was feeding at 3am and 7am but has now back-tracked to wanting to be fed at 2am and 6am. Although I manage to express 7oz at 9.30pm he will only take 3-4oz when my husband feeds him at 10pm. Is he starting to wake earlier because he is not getting enough at the last feed, should he perhaps be having closer to what I am actually expressing at that time?
A baby who is the same weight as your son and being formula fed would perhaps be taking around 4-5oz at the last feed. As your baby is having expressed milk which is digested more quickly than formula, and he is taking slightly less than the amount suggested, he has probably started to wake up earlier because he is genuinely hungry. It would be worth trying a split feed at 10pm and 11pm to see if you can increase the amount he takes. That along with having him awake for a good hour and a quarter may be enough to help him sleep slightly longer stretch in the night. Your son is also possibly going through a growth spurt, so if he appears to need longer on the breast during the day, then do allow him.
It is important to remember that when giving a baby expressed milk from a bottle, that they probably do not get the same amount of hind milk that they would if they were feeding from the breast, therefore they may not sleep as long a stretch in the night. Your son is also possibly going through a growth spurt, so if he appears to need longer on the breast during the day, then do offer him the amount he wants. But even feeding at 2am and 6am your son is doing incredibly well, and so are you for producing such an good amount of milk at that time.