OK so it seems as though the moderator can add a topic and reply to his own topic, but not others from the same forum..
So, I know you’ve said you checked permissions and couldn’t see anything wrong, but could you just check his permissions for the ‘Shop Talk’ forum?
The easiest way is to log in as him, and within the forum click Profile from the header, go to the permissions tab, find the ‘Shop Talk’ forum and click ‘View’..
Check to see if there is a green tick for both ‘Can start new topics in a forum’ and also ‘Can reply to existing topics in a forum’..
First thing to check!
On a side note, I notice that the forum is running UTC timezone – This should really be updated to a genuine timezone as UTC isn’t supported by WordPress. Not sure it’s responsible for this issue but it has been known to cause strange problems.