you can probably find more information in the rulebook at ...
this is what i found with a quick look. check with your gym for their standards, but if he wants to compete at 8y/o, as of his true birthdate, then he has to start sparring prior to that.
101.7. Classes of Competition – male and female
The age of a boxer desiring to compete in USA Boxing sanctioned events shall be
determined by the boxer’s age at the start of the event or tournament. The
exception is USA Boxing national tournaments, for which USA Boxing reserves
the right to determine the boxer’s age as of August 1 of the year the tournament is
Junior Olympic Boxer – minimum age 8, using the actual birth date,
maximum age 16.
(a) Age divisions and weight classes. The Junior Olympic program will
consist of four age divisions.
i. Bantam Division, ages 8, 9 and 10 years. Only competition at the
LBC club/tournament level is allowed, with the exception of nonadvancing
national tournaments. Advancement to regional or
national competition is prohibited.
1. Bantam Division (local) 8-year-olds can only box 8- and 9-year
olds at LBC club events.
: Any beginning weight, with five-pound increments.
Length of rounds
: Three one-minute rounds, one minute