bottle nipple size?

Kaylee • Case and Cole’s momma! 🌾🌾 #boymom
Okay my LO has been drinking 6 oz and now he doesn't drink but 5 oz and gets frustrated and it takes him 25-30 min to finish. I'm wondering if changing the nipple size would help. But then again milk still dribbles out of his mouth at times and he's already gassy. So just wondering what y'all thought about changing the size?