I dom't know exactly why, but I do know two boys who were like this, and they were at real risk for drowning. Unlike other kids who are at least trying not to drown (you still have to watch them closely, but at least they're also taking steps not to drown) these two boys could be pulled out of the water and go right back in, needing to be rescued again and again. Both are now adults but it was nerve racking to go to pools with them, because you had to focus entirely on them to the detriment of the other kids who might be there. One of the boys was a daredevil, and the other had sensory integration disorder, which usually makes kids unwilling to put their faces in the water.
The mom of the kid with SID theorized that her son didn't feel the same panic other people feel when deprived of oxygen.
I taught swimming and was a lifeguard for years, and never came across something quite like this.
Rockrose has a good point.
My other thought is that he may have realized the joy of being underwater and being completely weightless. It is something that I still enjoy to this day - to just "hang" there suspended in water.
Obviously, he needs to be watched. But you also might try teaching him how to swim underwater. In fairly shallow water place a toy or rock or some object that he needs to swim down to get. Also have him by the wall, sink down and push off the wall and guide to you. Once he can do that, then have him start kicking in a prone position, then doing the breast stroke.
Finally, and this is a bit scary - let him stay underwater a bit longer and see what he does. Chances are when he runs out of breath he will come to the surface. Or make it a game- if he can count - stay under till 3 and come up. Then go to 4, etc.
By the way, I am assuming he can open his eyes underwater. If not teach him by putting rocks on the bottom of very shallow water or get him goggles.
Let us know what happens.