cant blame him for atleast thinking about this, at 15 i always thought about running away and shit, i still want to and plan to live on my own in the woods somewhere for anywhere between 6 months to many years at some point in my life. but i realize it takes serious survival skills, careful planning, and not being 15.  im 22 and i dont feel mature enough to do it.
really just let him go, he wont last more than a week, and he will last a week and be fine.
to do this type of thing you cant just move into a known cabin that people will visit.  you need to go into the middle of fucking nowhere where people arent going to find you.  which also makes it a pain in the ass to bring supplies. it would probably take having an ATV to make trips and use for emergencies.  that and i think it would be ideal to have a canvas wall tent, with a wood burning stove for heat.  atleast one gun and plenty of ammo is a MUST, you need to be able to defend yourself from animals, and it makes getting food much easier.  he also needs to know how to keep and prepare meat, which is hard as shit, probably should have a small generator and like 30 gallons of gas for emergencies.  and an entire summer of hard ass work and preparation to get ready for winter.  you cant just go live in the wilderness at the start of winter and expect to be fine. you cant do that shit unless your a real fucking man.