D.W.
Breastmilk or formula is primary nutrition for the first year!!! I can't stress that enough. Food is only for practice. It will not and does not provide enough of what an infant needs for proper growth and development. You always offer breastmilk or formula then solids (usually an hour- hour 1/2 later).
And a hard bowel movement means there is not enough liquid in the diet which would make sense if he is only getting 4-6 ounces of formula per day. I hope you just left out some info for us and he is actually getting the proper amount of formula as advised by your pediatrician. Like Sarah D. I can't advise to amounts of formula as i exclusively breastfed. What does his doctor say?