I don't have kids but I remember being a teenager
not
too long ago. I'm 31 btw. Seems like your son was like me when I was a kid. Taking stuff apart just curious to see how it all went, but not remembering how to put it back. Trying to create new stuff out of old junk, I was in awe of the insides of a betamax player. LOL I am a creative, visual, hands on type person, do you think your son is this type as well? I did stuff like that, but my room was always a mess (always in mid project) I hated doing the dishes, etc, but was always working on something. If he is, I would not punish him for being creative or him trying to help, but punish him for breaking the shelf. Don't make it seem like it was YOUR project and yours alone, may make him feel shunned, like don't touch my stuff! Perhaps work on a project together, and show him how to do it correctly. Or if your dh can show him the correct way to service the lawn mower. Seems like he's gonna do it no matter what, at least show him the right way.
As far as punishment, I agree with rtsmichele take away some priviledges until you feel it's enough, and that you and dh have to work as a team.
How are the other two kids? Do they team up with eachother, and the youngest feels left out? My older sis and younger bro were always teamed up and ignored me, which is why I ended up taking all these strange hobbies and interests.