S.K.
This sounds like a normal transition of BMs. For formula fed babies once a day is common, for breast fed babies it ranges from many small ones in a day to once every few days. It is normal for babies to strain a bit when passing a BM. He is only constipated if the stools are hard and dry. If they are soft when he gets them out he is fine. The more breast milk you are able to give the less likely he is to have problems with constipation.
ETA: I disagree with with a previous poster about needing to choose one or the other (breastmilk or formula), give all the breastmilk you can and if needed supplement with rest with formula. Giving half and half is fine, any amount of breastmilk is beneficial for the baby.