To them, it's just playtime in a more structured environment. Adults view it as work. Many, particularly triathletes, feel pressured to swim better in a hurry. And, if they've been trying to swim on their own for several years, there are layers of bad habits on the slate to overcome. But there's one great equalizer.
Adults are rapt and absorbed pupils, far more dedicated students than are most kids.
And attention and motivation are the most important ingredients in learning new skills. Frankly, I'd find it very difficult to go back to coaching kids again because I've become spoiled by the quality of mature students and how much they learn in just a few days. They've had to carve precious time out of busy schedules to spend a few days with me, and they're intent on getting the most out of it. Just try telling him the only time to learn better strokes is when you're young. Get some coaching.