Hi K.,
My son had a similar problem, and my 88 year old grandmother gave me the answer. Add 1/4 teaspoon of corn syrup to the 8 oz. of formula until the constipation stops. Then you can cut back to 1/8 teaspoon. The idea is to counteract the constipating effect of the milk calcium, because formulas have more calcium than mother's natural milk, and no sugar from the mother's natural diet. It is better to be consistent with the tiny bit of sugar than to wait until the little one is so plugged up that it takes a laxative to break things loose. Baby intestines are very delicate, and cannot even handle much roughage at all without serious consequences. I read that one baby was eating canned green beans as a toddler snack food at 2 years old, and that caused intestinal bleeding due to too much fiber for him. Corn syrup is very gentle, and does not carry the possible baby health risks of honey, which is what was used back when my grandmother was a baby. Another thing I would try is adding more liquids to the baby's diet - maybe filtered or purified water mixed half and half with apple juice. Maybe give this instead of more solid or semi solid fruits for now until things get consistent.
The Best to you,
D.