Ok Jeremy - I am only going to mention this once because the good absolutely outweighs the bad, but absolutely think through what you are starting here.
Before you get your sons into cars/trucks and project vehicles, carefully consider how much you enjoy knowing where your tools, parts, supplies etc are. Consider how much you enjoy working on, and building
your own stuff. Think carefully about how much this hobby costs you for your own projects then consider double and triple the expenses - both in money and TIME.
Yes - you will lose tools
Yes - your parts and materials and supplies will just magically be absorbed into your sons project (and their friends projects). You thought you left the welder full of gas, you are sure you filled it just last week, but wait.... didnt your son just have to weld up a bracket for his buddies off road lights... and the tank was left open..... (yes it happend) of course they will deny it.
Now think about getting your sons into Soccer, Fishing, Academics, Volunteering,

g....
But as I said, the good far outweighs the bad. You will get to spend countless hours in the garage turning wrenches on his rig WITH HIM. This is time you get to spend hearing him talk about his stuff. Giving advice that he does not realize you are giving. No need to lecture, just talk. Find out who is hanging out with who, and which of his friends are good guys and which you want to watch out for. You will have those good guys hanging out in the garage too. You will watch him grow, mature, get stronger and more determined to complete his projects. Let him lead the way - with your guidance of course (no 9" lifts with 44's and the like) but if he wants to put dual shocks on her - let him. If he wants to paint it with rattle can paint, let him. You determine if it is safe and reasonable. Beyond this, let him lead. You might be surprised at how reasonable a teenager can be.
Never work on it without him. Ever. If he wants to get it done - then he needs to work on it too. If he is busy with school, homework or just catching up on the latest Hannah Montana episode - this is time you get to work on your stuff for a change - if you can find your tools. Just do not do it for him.
Have us come up to help, at times he will listen to us when he will not listen to you. Even though we are saying the same thing you are. Remember - really soon you are going to become the dumbest person alive in his mind - (come on, we all felt that way about our parents for a while didn't we?) Don't worry, that seems to change not long after the time they move out on their own and now they need advice.
Make sure he has some skin in the game, he needs to pay for some of the parts and supplies too. Son - you want a winch on it, well then you need to buy the winch and the bumper and the mounts (ok - Santa can help some)
Teach him wiring and connections please!!!! (if you doubt this, just take a look at the thread on Drew's bronco - oh and W's too) And make sure he understands why this is important.
And then truly enjoy spending time wrenching with your son. I still do.