While I don't shoot three fingers under, it stands to reason that the finger pinch with that method would be greater than split-finger, since all of the pinch would be occurring below the nock, with the worst of it placed on the ring finger. That's where the pressure/friction should be greatest and most likely to cause a blister such as the one your son has developed. He could probably use a Flemish release until his blister heals and then try a thicker glove for three-finger.