Can you get dry mouth from giving a guy oral sex?
Not normally. It's not clear why your mouth is getting dry. Some medications will do that, so it would be worth checking the side-effects of the medications you are on.
Like Ineligible said....but also, if your dehydrated, all that slobbering and drooling that usually happens while performing oral sex, might just empty out your salivary glands, and if you don't have enough fluid in you, they might not get replenished very fast. Try drinking lots of water.