What Causes Recurrent Itchy Blisters On The Heel?

My brother, who is in general excellent health, has recently developed large painful blisters on the edge of his heel. the first one was about the size of the tip of a finger, started deep, was itchy, then burst, but took longer than usual to heal (he is not diabetic). a few weeks later, another one developed then healed. and now another is coming up on the the other side of his heel. this is occurring on only one foot, and only in the heel area.