C.,
I am a lactation consultant so some of my adivice may not seem to apply to a formula fed baby but.....this is a baby we are talking about. The World Health Organization AND the American Academy of Pediatrics both recommend ONLY breastmilk (preferably) or formula for the first 6 months. NO cereal, fruit, cocoa or anything else. Babies will regulate their own intake to their hunger. By what chart is the doctor saying your child is "underweight?" The World Health Organization has come out with new charts for height and weight for both boys and girls. It is based on a breastfed infant which is the gold standard for human growth (formula is an artificial food so therefore the growth is not the same and shouldn't be used as the standard). These charts show that we have been overfeeding our babies no matter how they are fed. No wonder our society has such obesity problems (here in Georgia over 30% of our 3rd graders are overweight!)
My advice is to feed your child formula to his hunger, forget forcing the solids until he seems hungry for them and when you do start solids, give the baby real, nutrious food.....not that nasty tasting, library paste looking cereal! Offer bananas, mashed ripe avocado (great food-low allergy causing, good healthy fat, lots of nutrients and calories,), green beans, sweet potato..... If he is active, alert and meeting his developmental milestones, he is fine. Stop worrying about how many ounces......I guess that is one of the beauties of nursing, you don't know much the baby takes in and as long as the baby is healthy, it doesn't matter! You don't eat the same foods or the same amounts on a daily basis, neither does the baby!
Enjoy your baby, offer him formula to hunger, healthy foods when he is ready and don't focus on ounces of formula he is "supposed to drink" so food doen't become a battle ground that sets up life long problems.
Hope this helps.