(informal) A malechild prodigy, or (loosely) a talented male of any age.
1986, Don Belton, Almost midnight‎Daddy had been using that child for a boy prophet, building up a whole new ministry around him, since the child was four years old, calling him a wonderboy...
1992, James B Stewart, Den of thieves...the wonderboys of Detroit and Wall Street...
2004, Joshua Cooper Ramo, No Visible Horizon: Surviving the World's Most Dangerous SportHe could have been a robot really. He could be taught to do anything, the coaches thought. He was a wonderboy.