I don't want my boyfriend to come in my mouth...?

How do I tell him? We were having phone sex, cos it's long-distance. I don't like phone-sex and he knows that, but I do it for him. He was saying do you want to drink me? Now by this point he was really hard, and he'd just been saying how he wanted to "drink" me... I didn't want to ruin things, so I said yes. And he said how are you going to do that? So I said suck him hard... Bare in mind that I find this pretty uncomfortable, my boyfriend is 26 and I'm 18, and I had hardly any sexual experiences before him. I wouldn't say I would NEVER let him come in my mouth, but I need more time, I'm still trying to get used to giving bjs at all, and I have to say that I don't enjoy them anyway... He always grabs onto my head and starts pumping against the back of my throat until I'm gagging and if I pull away he holds it there. I'm also scared if he cums that it's going to hit the back of my throat and make me gag even more or what if it tastes really bad? I wish I wasn't so scared and I really want to make him happy and I didn't know what to say when he asked me, I would have ruined the mood. But I don't want to upset him and I don't know how to tell him... what should I do?
I don't want my boyfriend to come in my mouth...?
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