How to have clear consent in a relationship?

The guy I’m sleeping with is a very kind person and is great with asking me if I want to do things for the most part. But when I give him head he is kind of pushy. I have a bad gag reflex and I can’t do it for very long but he always begs and pushes my head down. I feel bad because I really like to make him happy but I don’t like the way he makes me feel when I give him head