Well, Moebius models has the Ironman Mk III and VII kits out (and a chrome Ironman Mk II), plus they just issued a War Machine. Those might work for a couple days as they are decently sized, have a few parts and given the modular nature of the Ironman suits, I wouldn't worry so much about the seams (perfect for a kid who doesn't like filling and sanding). Plus, I've seen them built up at Make and Takes at Wonderfest. They can be built relatively easily in a few hours if the right glue is used.
Plus, the paintjobs are pretty easy... metallic red and gold for Ironman and metallic black and silver for War Machine (maybe a light wash in the recesses to pop out details there). It might require some adult help to paint them properly, but he could do most of the work himself on them (if you trust him to be safe with using glue). And, since they are battle suits, one doesn't have to worry too much about painting faces.
After that, Polar Lights has a decent, yet more traditionally designed Captain America kit. There is also a reissue of the MPC Hulk, but I don't know if it is good enough for a young kid as older kits tend to have seam issues. Moebius also does an Ironman 2 Black Widow as well. I don't know of anything for Thor though, unless Toy Biz did one years ago as part of their currently discontinued Marvel kit line.
The way I see it, two figures could easily give the kid four days worth of fun and make a nice gateway drug to other models without going down the Snaptite path.
opcorn
