I went through the exact same thing with my now 7 month old daughter, who was also breastfed, mixed in with a little formula. My baby's poop changed about 4 times drastically throughout the first 5 months or so. Drastically changed (color, frequency, consistency). Without warning. It's all very strange. But now it's been the same since about 5 months or so. During one of these transitions, she didn't go for like 4 days; the doctor said not to do anything until Day 5 so we let her work it out. Apparently this is all extremely normal and occurs naturally. It should remain the same soon, as his digestive systems gets itself together!