Have you downloaded any add-ons? It might be an add-on that is unpackagedly correctly.
If not, try re-downloading from the
www.blockland.us source, which should be the same download as the one you got on your e-mail that gave you the activation key, if you used e-mail.
Remember, never download a game (especially independent games like this one) from another source other than the official one.
Hopefully you can get this problem fixed and start playing such a great game. Your son will have a lot of fun.