Firstly, this might not be for you. Sex should be shared and fun, l’m happy swallowing but it doesn’t mean everybody should be. The texture is similar to Greek yoghurt. The taste will vary from person to person depending on diet. Strawberries and water melon work wonders. It’s a no-no after asparagus!
Gagging is probably brought about in part by your own concern. Think ice lolly!
Secondly, I echo what others have said, but don’t take him back until he is ready to explode (you can feel the pulse change) and not so far as it’s beyond your own comfort zone.
Watch your own positioning, maybe try kneeling. If you are in a position where it would be difficult to drink a glass of water (or, let’s up the game, champagne) for example, it’s going to be awkward.
Take control, he really mustn’t thrust but enjoy being “worked on”.
Finally, make sure he is going down on you regularly. Now that is important.