Do you have a jaw thing? I forget what it's called.. Does your jaw sometimes 'click' when you eat and stuff like that? There's nothing you can really do about it (there is surgery, but that's a baad idea). Does the idea of having something to bite down on during the blow job sound appealing? Maybe you can find something for that.
I agree, to take break just use your tongue and hands for a while. Also, if he doesn't complain about it, and he'd prefer to get it deep with a little scraping than not get it deep at all, then don't worry (:

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