So, reducing the velocity of his stream this way will also reduce the velocity of the ejaculation, and will spare your gag reflex from being triggered. As you probably know, the first and second spurts are the strongest, then things taper off from there, so you can practice letting off sooner and sooner.
By reducing the initial inrush of cum, you will be able to more easily concentrate on getting used to the taste, texture, and the feeling of the cum as it flows into your mouth slowly and gradually instead of getting it in a sudden deluge. By having smaller amounts come in slower, it is more quickly diluted with saliva and depending on your progress here, much easier to swallow. As you practice, you will find yourself becoming more and more eager to have him cum in your mouth with strong manly spurts. And you will also eventually be more and more comfortable with being a genuine sweetheart and letting his cock relax and soften in your mouth until he is ready to remove it.
When you've finally leaned to go "hands free", you will love and crave the unique and sexy feeling of your lover climaxing in your mouth. And you will have the satisfaction of knowing you gave him a blow job worthy of a porn star.
Regarding the gag reflex. Learning to tame it won't affect you too much. But if you don't really need one as much as you did when you were a child, your next lesson should be to deep throat, since it's related to cock sucking.
Like any nerve, it gets used to a certain stimulus and eventually treats it as normal. So start by going deeper on the cock until you start to gag, then back off, then immediately do it again and again until the reflex gets tired. It's as simple as that, and you already have a head start from the exercise above. But over time, you will diminish the gag reflex to the point where it really doesn't work effectively anymore. The only question then is "do the the benefits outweigh the risks".