DS is about 4 1/2 months... I just feel like he doesn't eat enough, though he is definitely gaining the weight he needs. He's consistently been 85% for both weight and length... he's 15 1/2 lbs. But my letdown is SO SLOW and he doesn't nurse nearly as long as he used to... I can tell he doesn't get tons of milk per gulp from me... even when I occasionally feed him from a bottle I can tell he gets tons more from the newborn flow nipple than he does from BF from me... And when I do occasionally give him a bottle he only drinks 2-3 oz at a time even though he's close to 5 months old... and since he doesn't nurse as frequently as he used to it just worries me.
I would get access to a scale and do a feed and weigh to see how much he actually eats. Our pedi was concerned that BF for 10 minutes wasn't enough, but DS was eating 3 oz in 10 minutes. 2-3 oz sounds ok because from 1-6 months, a baby eats 19-30 oz a day, so with eight 3 oz feedings, that's 24 oz, right in the middle. But as long as he's gaining weight and you're seeing enough wet diapers, you shouldn't be worrying.
Babies can become more effective at the breast and nurse less while still getting what they need. Like pp said, if you're getting enough wet/dirty diapers and baby is content after feedings then there is no need to worry.
Re: eating
This.