DCD,
Great plan! Nothing like plannin' ahead! Hope your son's interest continues to develop.
Check out all the different R/C magazines out there as well. I just read an article on the Hangar 9 "Easy 2". Holy cow,[X(] a screwdriver and an adjustable wrench is all thats required to get it ready.
My personal opinion, for what it's worth and I have to agree with woodbutcher as far as buying it now, is to buy an ARF that takes a bit more time to get it ready. You're gonna' find yourself making the occasional repairs after you get airborne, it's the nature of the beast (and what an awesome beast it is!), so you might as well get used to mixing epoxy, gluing your fingers either together or to your plane with CA, repairing covering and all the other good stuff that goes along with this great obsession NOW as opposed to learning it in the spring. You can also spend some time learning all the facets involved with two stroke glow engines. Spend this winter breaking that new engine in! Why wait 'til spring?! Not to mention, every time you and your son look at your creation it will bring you closer together as well as create a burning desire to get it airborne. Then, as they say, the sky's the limit.
Best of luck to you!
Lastly, the worse possible case as that you and your son don't like it. then you just unload all your stuff on ebay at close to retail prices!