Here you go! As long as it's not a sawed off, or a handgun, then it looks like he's fine

https://docs.legis.wisconsin.gov/statutes/statutes/948/60

(1)  In this section, “dangerous weapon" means any firearm, loaded or unloaded...(etc)

(2)  (a) Any person under 18 years of age who possesses or goes armed with a dangerous weapon is guilty of a Class A misdemeanor.

c) This section applies only to a person under 18 years of age who possesses or is armed with a rifle or a shotgun if the person is in violation of s. 941.28

941.28 No person may sell or offer to sell, transport, purchase, possess or go armed with a short-barreled shotgun or short-barreled rifle.