If he is posting through a network that has a submask that can explain it. I had a bunch of friends who lived in one house and they had to decide how to take certain 'tests' on an old forum I frequented because it logged the IP of anyone who took the test and only one of them could take it. Eventually they figured a way around it but I forgot how. On the forum they all showed the same IP address in their posts.
If you trace the IP online you should be able to determine if it is a network or an individual machine assigned to that IP.