user dimmed with checkmark

I have an iMac running Lion OS. I added two additional users for my two sons. I recently wanted to change my own password, so went into the users and groups panel and clicked the padlock. My younger son's account and the guest account,as well as my own (admin) account became selectable, but my older son's account name remains dimmed with a small orange checkmark on it. I believe he changed the "short name" on the account since I had originally set it up with his and his now ex-girlfriend's names on it. Now it says his name alone. Any thoughts? He is into some things online that we need to monitor and I don't like being denied access to his account. Not sure if he did this on purpose or if it was a result of removing his ex's name...