Afaik there is no state where an underage person is allowed to handle assault style fully automatic weapons without the supervision of a parent or legal guardian. So even if it is his gun, it falls under parental supervision until 18 years old.
Semi automatic or hunting rifle is a little more murky.
Anyway I think that your comment nailed it - a person under 18 was able to obtain and handle a gun without supervision is a failure to secure.