Re: Should I supplement, or give solids?
From what you've told us, you are pumping enough, and your baby is eating an amount that's similar to other infants. Most infants in daycare take in about 1-1.5oz per hour that they are separated from their mother, spaced out into feedings every 2-4hrs.
What size bottle nipple are you using? Perhaps switching to a slower-flow bottle nipple would help meet his suckling needs?
Or maybe he has other needs, like just to be held, or rocked, or sleep, or dry diaper, or any of the other needs that infants have but can't articulate? Offering your mom some ideas for soothing him that you know work (rocking, going for a walk, being worn in a sling, etc), instead of feeding him, may allow you, your mom, and baby to slowly realize that he really is getting enough.
You asked about offering cereal - solids generally aren't recommended before about six months of age, when the infant shows signs of readiness. Here's a quick article about when to feed solids: http://www.llli.org/FAQ/solids.html
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