Sucking it

So when I sucked his dick for the first time it was hard. And not just his dick. It kept hitting the back of my throat and didn’t feel good. My mouth kept getting full of cum and I didn’t want to swallow so I kept having to stop, it wasn’t very pleasurable. But he said that it was the best head he’s ever had. I want to tell him that I don’t like doing it, but he gets so excited when I’m about to do it. What do I do???