My son will be 11 months on the 12th. We weaned at 10 months & he gets anywhere from 15-25oz of formula a day plus about 15-20oz of "real food" plus some snacks. He's a big boy (23 pounds) and a good eater.
Is your son eating a lot of solids at this point? I think the amount of formula totally depends on the baby, but I do think if they get full during the day it's easier for them to sleep longer at night w/out needing food. However, sometimes my DS will go two or three nights in a row without eating & then have a night where he has a screaming fit til he gets a bottle? I just try to roll with it as best I can

Good luck!