both my girls were not even 18 lbs by that age. They didn't hit 18 lbs till they were 18 mos. So I totally understand your anxiety about not getting enough calories.
That being said, I must tell you that once I cut back on their liquids they ate much better and gained weight. So, I would say keep trying. Can you flavor the formula? I don't know if you could get something to add to it, maybe like they flavor medicines, to make it taste better in the cup.
Kids that age should only be getting 16 - 20 oz of milk a day, so I think the same would be true of the formula. More than that and they stop eating food cause their calorie needs are met already.
N.