The gag reflex is the least active first thing in the morning (unless you're hung over from the night before). Try giving him a blow job when you first wake up. This is the best time to practice with a minimum of gagging, and let's face it, very few guys are going to argue with starting the day by getting their dick sucked, especially if they have morning wood. Once you have had a few successful attempts, you'll become more accustomed to swallowing semen and the gagging will stop.