DS is 5 weeks old and I give him 4 ozs at each feeding. He eats every 3 hours and almost always finishes his bottle.

He's still a little fussy afterwards.

How do you know to increase the amount of formula he's getting?

I saw in Baby Whisperer that 4-6 week olds eat approx. 4 ozs and then from 6 weeks, it can increase from 4-6 ozs.