I suspect he doesn't know how to access his Private Message Inbox as his is full.
For Kingspriest: click on flashing icon on upper right corner of your home screen then see your PM messages. You'll need to delete all that are unimportant to clear for new PMs.
He's a new member - he can't PM until he has the minimum Forum post count to be able to send/receive PMs.
The default message is "over limit" to anyone trying contact a new new member.