Don't worry, your son's obsession will go away, I was obsesssed with lot of things like farms (4-5 years old) cows (5-7 years old) , thomas the tank engine (5-8 years old) , furbys(8 years old), Powerpuff Girls (8-9 years old) Robin Hood, Kid pix and the Flintstones for little while, Muppets(9-11 years old), furbys again (10 years old) , frogs (11-12 years old) till SpongeBob Squarepants (12-13 years old) Robot Jones for a little while, now my current obsession is Doctor Who and David Tennant. They effect my learning except the little while ones.
other current little while ones were Honda chasing Angela Anaconda, Flying Robot Steam train chasing Robot Jones and Helecopters chasing Doctor Who, then wind turbines, now green vampires.