I'm only aware of the one dummy Big Boy model which I believe was originally offered by Monogram, and which has subsequently been offered under other brand names. Is is a pretty good static model, although I would rate it below the much older HObbyline static model of the Nickel Plate Berkshire. The most glaring problem with the Monogram Big Boy to my eye is the handrails on the boiler. The are cast as a 1/8" high plastic ledge running the length of the boiler, with no clear air between the bulge representing the handrail and the boiler jacket. The old HObbyline kit used separate handrail supports with wire handrails, which looked much better. The other issue is more minor - the Big Boy valve gear is cast as a single rigid piece, so that when it is attached to the wheels and side rods, it prevents them from being rotated to other positions.
To its credit, the Monogram model was considered good enough by many scale modelers that Bowser made a conversion kit to put metal wheels with electrical pickup on the plastic Big Boy tender so it could be used with the Bowser die cast UP Challenger and Big Boy operating models. I remember visiting a hobby shop on Woodward Avenue north of Detroit many years ago where one of the employees had his girl friend (also an artist) paint and weather a Monogram Big Boy. She did a super job. They had it on display in a cabinet with their other brass models for sale, and I remember having to look very closely to finally realize that it was the dummy plastic model.
By the way, not to make more work for you, but as long as you are considering a dummy HO Big Boy to satisfy your son, the old HObbyline Nickel Plate Berkshire dummy model is virtually identical the Pere Marquette Berkshires. It was preserved Pere Marquette No. 1225 that was the locomotive used in making the "Polar Express" movie. The HObbyline kits show up regularly at swap meets at very reasonable prices and I expect on ebay as well. Geezer
Revell and Monogram are one company now, and you are correct that the Con-cor model was a rebox of the Revell/Monogram kit. I built one this past summer and it is a glue kit, not a snap-together kit. As I recall it was about 20 dollars or so. With the right amount of painting it makes a decent model. Carve off some of the cast on details and replace the handrails with wire and you'll have a decent static model. The valve gear is pretty neat and the price is right. I have one of the old Hobbyline Berk's that I'm going to get around to eventually. I notice that Revell of Germany has some really neat European locomotives. Trumpeter is coming out with a 1/35 scale German steam engine this year that should be a real wow! If you ever get to the point where you think the level of detail in model railroading is better than ever(which it is...), take a look at some of the stuff that military and boat modelers do. There are photo etch sets for practically every model.
Thanks for all the suggestions/comments. i think I will go to one of the big local hobby shops and see if they have any. Otherwise I see them reasonably cheap on ebay. Seeing them in the store may help me decide too.