Baby only nursing for 5ish minutes :/

Rebecca
I'm a FTM- my milk recently came in at about 4 days. Baby used to nurse 20-40 mins. Now, he's only nursing in very short spurts- sometimes as short as 5 minutes. This'll last him anywhere 30mins-2 hours. It varies. I worry he's not getting enough. He doesn't seem to be pooping quite as much either. How can I entice him to stay attached longer? When he pulls off my breast he has no interest in re-latching.