I need some additional information.
1. Was he born with this, or did it appear later?
2. How long has he had this white spot?
Such a spot is most likely caused by a fungus of a variety called tinea. However without physically looking at it and taking a skin cell test it would not be definitive.
The skin test is referred to as a KOH (potassium hydroxide) prep. IN this procedure a small numbrer of cells are taken from the advancing edge of the area by a method that does not cause pain or discomfort. Your skin has lots of dead cells laying around on its surface. A few of these are used to look for fungal spores.
YOu should go to a physician to have this looked at and to verify if it is a fungus.