Awkward question? .... how do i get him to stop putting his hand there?

I'm eighteen and my boyfriend's twenty, we've been together about two months and before him i'd only ever slept with one other person, he's had a lot more experience than me. He's lovely, a real gentleman, hilarious, good looking.. he like dotes on me which is lovely. He always picks up the bill, he drives me miles, he's so caring he wont even let me walk next to cars on the pavement, he's even paying my car insurance at the moment! The only thing is that when we're getting down n' dirty ;), when i give him head he'll always put his hands on the back of my head and be really forceful.. i know that that's what does it for some guys, and obviously him as well, but i mean it makes me gag and its really uncomfortable.. he does it everytime. I don't want to seem like i'm immature or anything, but how do i ask him not to? I guess i'm just scared of disapointing him because he does so much for me..