2days with out pooping

My son is 4weeks old, he was breastfed, but I stopped due to my milk decreasing... He was in enfamil newborn an was doing fine all of a sudden he started getting constipated, we went out an he had a poop explosion an pooped again.. it was like a greenish pasty color.. but irs been about 2 days he hasn't pooped. He'll pass gas an maybe a tiny bit comes out but not enough... IDK what else to do the pediatrician told us to give him an oz of prune juice but no luck :(

Help!!!!