Lots of good advice already on here. I particularly agree w/ the person that said help expose him to a lot of different things. We ISTPs are stubborn so if you push too hard, you may get rebellion. I think it is probably better to passively make suggestions or if he expresses an interest in something, be supportive. I am an ISTP - I have a LOT of very diverse interests; I am naturally very hands-on - anything to do w/ tools or making or fixing things with my hands I naturally excel at; I like sensory experiences; I am only interested in solving practical problems with real life applications; I can work endlessly on something I am passionate about yet will cut corners when I am not; I enjoy working with people who I think are smarter and better than me and have little respect for those who aren't. I have a B.S. degree in mechanical engineering so don't automatically rule out 4-year university just because of his type -- it is possible, but I think it would help to be majoring in something practical and if the real-world application is not real obvious as in a trade, that somebody explains the connection at the outset. I personally think I would have enjoyed trade school more.
If he likes electricity, he might like being an electrician. This might involve studying at a community college or trade school then apprenticing under a licensed electrician so that he can get his license. He may also like welding, plumbing, construction, or another job working with machines. He may like cooking (I do).
He may like mechanical engineering. He may be able to work for a sports company designing and/or testing skateboards, surfboards, skis, exercise equipment, or other products. Being interested in ME, he may become interested in materials science - skateboards are typically made out of wood but could be made out of composite materials as well. I also saw a video of someone who made a motorized skateboard for a senior design project. With a degree in engineering, maybe he could design sets for Cirque du Soleil shows or do special effects for movies or go on to host a TV show like Mythbusters.
Speaking of TV shows, I am betting Mike Rowe on Dirty Jobs is an ISTP as his job is to get a taste of everyone else's jobs. He might also like working with food. I'd totally work for Ace of Cakes if I had the skill set.
He may consider being a stuntman. A farmer. A pilot. A diver. Underwater welder. Photographer covering news, wars, sports, or controversial issues.
I am reading a book called "Do What You Are" which is entirely about personality types and lists possible careers. I am also reading The Pathfinder which is a more holistic career book that your son may find useful one day.