If giving Formula after solids... they will be too full to have Formula or their breastmilk.
Our Pediatrician said, for the 1st year, to give breastmilk/Formula BEFORE solids... and on demand. Otherwise, a baby will 'wean' from it. And for the 1st year of life, breastmilk or Formula is a baby's PRIMARY source of nutrition... not solids and not other liquids.
And appetites... do fluctuate. And per growth or growth spurts etc.
Ask your Ped, how much Formula he should be getting for his age.
I breastfed, so this is not your case per say, But I nursed on-demand.... primarily. And solids, was not the 'main' meal so to speak. Also, I did not give solids, 3 times a day... but gradually worked up to that frequency... for the 1st year. Which is what our Pediatrician, also recommends.
Both my kids, were born over 8 pounds and were always in the 95th-97th percentiles, since birth.
all the best,