The phrase "boy king" refers to a young boy who holds the position of a king or ruler. It suggests that even though the person is young, they have the power and authority to govern a nation or kingdom. Full definition
After a treacherous coup, Evan, the young boy king of Ding Dong Dell is banished from his kingdom. (digitalchumps.com)
A treacherous coup has forced Evan Pettywhisker, the young boy king of Ding Dong Dell to flee his kingdom. (otakudome.com)
The 1998 World Cup final was supposed to signify the coronation of a new Boy King, placing Brazilian striker Ronaldo at the summit of the game, assuming his place among the pantheon of the greats at the culmination of a tournament in which he'd shone as brightly as everyone had hoped. (footballwhispers.com)