I gag when he gets close to finishing

I have a relatively low gag reflex and love giving my bf head, but whenever he gets close to finishing I start gagging more easy. I'm not grossed out by cum, but I think the idea that it will happen without me knowing when or hitting the back of my throat throws me off. I feel bad, it can make it hard to keep going.

How do I fix that? He already says when he's going to cum, and says he's getting close and such but I still gag.