Based on my experience as a homeowner and people I've known who've gone into trades , I'd say either a plumber with some sort of certification or whatever papers separate better plumbers from the rest in whatever state he lives or works in, or an HVAC tech with similar certifications. I believe it would be worth the time and expense to get the schooling and certification in a skilled trade.
A couple of years ago I had a piece of property surveyed. Field surveying seems like a good trade for someone willing to work outside year round, who can put up with heat, cold, ticks, steep ridges, etc. Opportunities to work in both rural and developed areas. Based on what the survey cost me, it must pay pretty well.
He might be able to work for a contractor in whatever trade(s) appeal to him as a helper, gofer, or whatever to see what it's really like. If he has a strong work ethic, and is willing to learn and take on more responsibility and difficult assignments, the future will take care of itself. He has to make himself a valuable employee, because that's who a contractor or business owner will keep when business slows.
