It sounds more like he's a bit impatient that the nipple doesn't let down as quickly as what he gets from a bottle. Also, with a bottle, he really doesn't have much choice but to drink fast, it's coming into his mouth much more quickly. If you are supplementing him with formula, this can also affect your milk supply, so he may not be getting as much as he needs from you.

Can you give us some more information about how often he's bfing and bottle feeding and how things have gone over the last few months? That might help us give you more advice on encouraging him to bf.