Make a list of those foods which require sips of water and then try to avoid them. My list for years included white rice, bread, white meat chicken, cake, popcorn, etc. Always make sure he has liquid prior to any snack or meal. I once took water from a restaurant patron when I ate before I had my beverage. Not a good day.
Be hopeful about the pet scan rather than nervous. Odds are it will be fine. However, scar tissue often shows as cancer so hopefully whomever reads the scan knows what they are doing. Remember, half of all doctors are below average. Courage.