Turns out the ball is a Tru Flite. USGA rules are that a ball weigh no more than 708.8 grains, so I suspect this one is pretty close, minus the material drilled out. Obviously there is some extreme FOC on this weapon. Also remember the dimples on a ball help reduce the drag. Not sure how much it helps in this application. Just for fun, I looked up the speed at which a golf ball leaves the face of a club. The article I read says a perfect center hit from a typical 100mph swing from a driver will impart an initial velocity of 148mph. 148mph equates to 217 feet per second, so certainly faster than my brother's recurve, but not that far off.
Given all the rocks in the area, I'm amazed it didn't bounce off one and return to hit him in the head.