Your son got a hacking tool Nodus or did some modifying of minecraft files. First thing to do is MAKE A BACKUP!!!
For single player(If your on a PC) Go to %appdata%. Click on minecraft and If you see a folder called "nodus" delete it. Then delete your bin.(This will delete any mods/jars installed.) After that, open up minecraft and at the login page click options "force update", log in. You'll see it download a fresh jar file. Now reinstall your mods.
If its Nodus. Press "u" on your keyboard then type fly mode creative. This will only work if you use Nodus.
For multiplayer Servers: Check your plugins!!!
Since new minecraft 1.5 has been released it could be a bug of somesort. :l