St. Nicholas. From his cradle he is said to have manifested marvellous indications of piety, and was therefore selected for the patron saint of boys. (Fourth century.)
The custom of choosing a boy from the cathedral choir, etc., on St. Nicholas Day (December 6th), as a mock bishop, is very ancient. The boy possessed episcopal honour for three weeks, and the rest of the choir were his prebendaries. If he died during the time of his prelacy, he was buried in pontificalibus.
Probably the reference is to Jesus Christ sitting in the Temple among the doctors while He was a boy. The custom was abolished in the reign of Henry VIII.
Boy in buttons
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894