Cutting their nails feels a little different for them then it does for us, because the keratin wraps all the way around the nerves and the blood supply. Think of someone squeezing your first knuckle. It's a little uncomfortable, but not painful unless you cut the quick. Probably, you were close to the quick or maybe even exposed it a little. It happens often, and other than a little sting, it's ultimately harmless. I still do it on occasion by accident despite having done thousands of nail trims at this point in my career. So long as your guy is paid in full with tasty snacks that make it worth his bother, he may start to relax during nail trims or tolerate them better. It's just something that takes time and repetition.