Hi Smidgin and Wecome,
You may have seen me post this elsewhere, but babies are infinitely better at removing milk from the breast than pumps are. If your baby is nursing well and gaining well, there is no need to worry about pump output. How are your son's wet/poopy diapers? A healthy breastfed baby should have 6-8 wet cloth diapers (5-6 really wet disposables) and anywhere from 2-5 bms per day. Sometimes babies your son's age have fewer bms per day, but volume is important. If he is only pooping every day or two, they should be big ones.
Sometimes infant cereals can decrease a baby's appetite for breastmilk. This is because cereal is not digested quickly, and can make baby feel full longer. Many moms find that foods that are more nutrient dense and easily digested are a better fit for their breastfed babies. These include mashed banana, avocado, and sweet potatoes.
I hope that helps. Let me know!