Attempt to create a new user on your forum. If you have no problems, try one of these:
Ask him what name he is trying to register with.
Is he trying to use special characters in his desired user name? And is your forum is set to only allow regular keyboard chars.
Is he using a banned email address, or an invalid email address.
Is he blocking cookies on his side? He may have installed a "Privacy Protection" add-on to his main browser. Have him try another browser.
Check StopForumSpam for his IP address
His issue may be a PEBKAC error, or a misconfigured setting with your forum -- have you updated anything recently? Check your admin log
