First, I applaud you again for your efforts in breast feeding! Each feed should vary - and with the breast, you should wait until he pulls away - he knows when he's full, it's a great natural thing! Here's what my ped gave me as a way to gauge what my son should eat per day (daily ounce/bottle average)
multiply 2.5 x his bodyweight. that total is how much he needs to drink PER DAY in ounces. so - if he feeds 4-6 times per day, divide the total by how many times he feeds, and thats your average per ounce.
Call your ped - have your ped validate this - and I urge you to make your dear MIL aware that YOU are in charge of your childs best interest!!! Don't let her overfeed him!!! Bless your heart - I totally feel your pain... my MIL is the same way!!!
http://www.babycenter.com/refcap/baby/formulafeed/9136.html
another great resource - babycenter is so great!!!