You can't even buy an SBR without an ATF tax stamp, and if this were an illegal SBR, this trial would be waiting for the ATF to be finished with him. Keep in mind the current average wait time on a tax stamp is >200 days. As someone <18 (who therefore does not meet eligibility for the SBR tax stamp) the firearm would be illegal and he'd be tried under federal law, serving a up to 10 years (and fined up to $250,000) upon conviction..
This wasn't an SBR, it was an off-the-rack AR15 variant semi-auto rifle.