Advice on deep penetration-pain?

Hii! So this is super random but when my bf and I have sex (doggy) it feels good but when he fucks me deep it hurts? It feels like my stomach is in pain, and I want to be able to take it in deep. Do you know what I can do to make it not hurt so much during doggy? And why does it hurt everyone he goes in deep? It’s not like he’s a 9 in so I feel like I should be able to take it in?