Re: Need help to fatten up baby.
Before doing anything else, I'd find a lactation consultant and do a couple feed/weigh to see how much he's getting approximately per feeding. The lactation consultant can also check him for a tongue tie as pp suggested.
Formula, might not be your easy fix. My DD has been dx'd as failure to thrive since she was a month or so old. She's never breastfed (long story...), so has been on pumped breastmilk. I share this-as feeding via bottle doesn't necessarily fix the issue.
Did your pediatrician make a referral to anyone else-for example GI or cardiology, to rule out possible issues that can present as poor weight gain?
Just my 2cents..
Mom to Mason (3/2003) and Maddie (11/2007) and Morgan (3/2011)