Dave,
Good advice so far, and I'd like to add the following to it.
Just like flying, sailing takes a bit of getting used to. You can't learn to fly over night, same for sailing, except much quicker, sort of. Teaching the brain/muscles those new reflexes takes longer than figuring out what to do when something happens (make sense?). It isn't that hard to learn. If I can teach myself how to sail, then anyone should be able to do it (and with a little help, do it right to start with! - LOL).

Wood/plastic/foam/metal or a combination of all of them, whatever you are comfortable with using. Each have their own 'pros-n-cons', all of them can produce a very nice boat. Unlike typical flying, weight definitely does have a use in boats, especially sailboats, so building with sort of heavier stuff is okay (to a point). Wanna 'show-off' boat for the mantle? No problem! But, that 'show-off' boat would probably sail as well as any other one.

Starting with something sort of simple is certainly one way of doing it. Not that the actual sailing will be all that easier/harder, but it's sort of easier on the wallet - lol. Diffrent boats have different characteristics, just like planes. Transitioning between different ones just takes some thinking and attention. Unlike planes, things happen sort of slower with boats, so changing boats isn't all that difficult, sort of. The "Oh Sh**!" thingys are still there, they just aren't as 'fatal' as with planes. Besides, that's half the fun of it, right?

Something else to think about is to select a boat that you (or your son) likes. That can be sailed where you have access. Isn't more trouble to transport than it's worth. And that doesn't require huge amounts of building ability, and/or money. Expect stuff to go wrong, it will. Does that remind you of your first plane? It should, same basic thing.

The 'mainest' thing is to enjoy it, even the mistakes. It may sound 'daunting' but it certainly isn't impossible for the average person. Your experience in building and flying will definitely be a benefit (even the mistakes, believe it or not). Remember to turn the electronics on BEFORE putting the boat in the water! that's the biggy!
Have fun...
- 'Doc

[carry aspirin. thats for those "Why did I do that?" thingys. Willow bark is one source for aspirin, but gnawing on trees will get you into more trouble than you'd ever believe.]