It's probably not the glove, but tender skin. He may need a thicker glove, and or an deeper hook on the string, better drawing arm alignment, etc. If he plucks on the release, that will also contribute to blisters building up under the glove. A lot of shooting can lead to callouses, but if one doesn't stop in time on individual shooting sessions, you get blisters first.
Hard to say what's causing it without knowing where the blisters are, something about the gloves being used and the string (strand count) and serving material thickness.