Baby Chokes

Hello

My son is 6 weeks old now, started with a slow flow nipple in the Nicu and upgraded and then switched to the Mam for discharge, it’s the only nipples now he seems to like. That being said we noticed he was tiring himself out on the 0 and drinking less breast milk, so two weeks ago we gave the size 1 a shot, at first things seemed to go well, but slowly he starts to grunt, wiggle and of course choke. On top of all this he has been basically vomiting/spitting up more then usual to, a few times it seems like half a bottle! Any experience or ideas on what to do?