My 6 wk old's poop not normal. What do I do?

My baby has been constipated sinse the day we brought him home. at first he was only going twice a day while breastfeeding. Very fussy. after switching to formula at 2 wks due to problems on my end, it got worse. Once every 4 days. Doc said to give him 20 ml of pear juice and water. but cant do that all the time. Switched to sensitive formula. Now he has a bm every 2 to 3 days but it is still very hard and he cries when he poops. The past couple days he has been very fussy. He cries durring feedings, He cries basically anytime he is not sleeping and he is a very light sleeper, often wakes up and fusses to pass gas, then back to sleep. Last 2 bms have been solid with some runny or normal poop. What does this mean to have both normal and solid? Last night he screamed for a couple hours straight before falling asleep and then was fine until this morning and now wont let me put him down!