Normally babies his age drink between 19 to 32 oz per 24 hours. And this amount stays more or less same till they are 6 months old. How much he is actually taking at one feed is hard to say but I would probably prepare a few bottles of EBM 3 oz each for his caregiver. He may easily take a bottle from another person once he realizes you are not around.
As for nightwakings, if you are thinking he is really hungry, and that would translate to ravenous and longer feeds, I would feed him. THis age is typical for distracted feeds during a day and making it up at night. He will grow out of that, but yes, I can commiserate with you for what you are going through right now.
There is also a thing, if he is going to have bottles, he may be drinking and be "active" at the same time, he may actually increase his intake because he won't be tied down by nursing, just a thought.