Tim,
Buy him the wood, and let him play, develope imagination, and hand/eye coordination. He's too young for "video games". When he gets tired of "the characters"(Thomas, and Brio) There's Whittle Short Line (wood trains, compatible with the wood track system, that
arePainted to represent current rail lines. They also have Metra trains). Also, don't glue down the track, it's like a puzzle. He'll get frustrated, but will eventually figure it out (and maybe be able to design a layout someday, without having to ask someone else to do it for him).
We humans are engineered to learn, but we need teachers,... and learning curves to fight with.