Newborn sleeping too much

H • •🌈💙12/18, 💙11/19, 🌈💗11/21, 💙5/23•💍03/13•

My boy did super good yesterday and ate 25/35 minutes each session, now I’m lucky to get him to eat 15 per session. He’s super sleepy and hard to wake up. He’s 12 days old and I’m super worried he’s not gaining weight. He doesn’t sleep longer than 4 hours but I was told 2 hours. What do I do ? I’m about to give him formula, I’m losing hope.