Hi Marlene!
It is really hard to figure out what they are going to eat at any given time isnt it!
Has he taken a bottle before? I think the best thing I found was to do small batches at a time. They may not drink as much on the bottle as the would on teh breast, Charlotte doesnt.
Are you going to express to feed fresh, or freeze it? I didnt feed DD from a bottle until she was about4 weeks, and then not very often cos she (still) doesnt ike them, but if its fresh it is much easier than if the BM has ben rozen. It hink freezing it can change the taste

Anyway I digress! lol Usually I expressed only about 60-80ml (per feed) when I left DD with my mum (she was about 2 months) and she would drink all but the last 10 mls
I think it is about trial and error though. But small batches in the bottle at a time will allow you to not waste as much

Hope that was of some help
GL