I don't know of any way to block every one not on his contact list, but there's probably an app for that. :-)
However, in the mean time you can go to... Settings---> Notification Center---> Messages---> and check Show Alerts from My Contacts. At least this won't allow awful messages to be displayed on the phone while locked. And someone else sees it while it's on his desk and get offended by it. (Could lead to punishment to him with the way the schools are now a days. That's another topic...)
He'll still receive the messages, but like I said, at least they won't be displayed on the lock screen. He could delete the message in iMessages with out having to open the message up too.
In the mean time, block that number.
KOT