It was good that you nursed your baby exclusively for first 2 months. Since you are working mother, you could not continue the same. So the option available for you is milk supplements.
It is natural for babies to have alteration in their bowel movements when feeds are changed or new feeds are introduced. If there is constipation
, recheck the way you prepare the formula feeds.
You can administer supplements which are low in calcium if the problem persists. Also ensure that his urine output is adequate.
If your baby passes stools once in day, it can still be considered normal and you need not worry. Do get your baby reviewed with your doctor if the condition does not improve.