I would definitely begin with fairly simple day hikes and go from there.  What in the outdoors appeals to him?  Build on those interests and let him take the lead.  If he does not have any particular interests, and at this stage of life he may not, expose him to as much as possible, keep it varied and assist him in investigating various appeals.  Be a safety watchdog.  At this age, safety is not usually a big concern, but he is an individual and we are all different.

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