You're right, I can't see any difference between the two extensions either. I assume you've tried forwarding to different outside numbers? Can you forward that extension to another extension? Do you dial the double 9 when just making calls from any phone? (You could program your ARS tables to insert the 9 for you).

If you can't find the solution, you may just want to change him to another physical port, and renumber the port to his old extension number.