I don't know/remember what the prices were like at Hobbytown in Oak Park, but Antonius and I were recently at a store on Irving Park which was having a going out of business sale on everything, including model trains and accessories. I couldn't believe how expensive all the little bits of paraphrenalia were, even marked down. So I don't know if this is the route you want to go for your cake decoration.
How about a few "buildings" constructed out of Legos? (The big Lego size has some Bob the Builder sets, you could put Bob and friends on the cake, too.)
Another point to keep in mind is SHORT ATTENTION SPAN. In two weeks your son may not remember that he wanted buildings too!
(Our son had requested that I depict the entire solar system, including a comet, in frosting on his cake. Luckily he forgot about this idea by the time his birthday actually arrived.)