I don’t think the kid will actually serve it out in prison though. The primary issue here seems to be that the kid is actively saying they will commit more crimes. No matter the crime, you can’t really give a light sentence to someone telling you, in a court of law, that they will do it again.
The ideal would be that they rehabilitate him until he’s able to be safely released with no fear of repeat offense, or at the very least, until the kid is smart enough to lie.