we had "cent" last week. Took some explaining. Not sure he'll be using that often.
What my boy finds a bit confusing is they don't stick with a particular pattern each week.
We've hit on the best way to help him to learn them, although I won't win any parenting awards for this one. He loves playing with his dad's ipod. So we change the password to start the ipod to a different spelling word every day....works a treat!