PPTP VPN and MAC addressing

I have spent the last two days searching for answers. I have a rather odd situation that I am trying to work with. I have a PPTP vpn server hosted on Win 2008 R2 and I am able to connect and browse the internet over the tunnel, however my problem is that I have an iboss web filter that relies on MAC addresses to apply the correct filtering. So when I connect, the client shows up on the iboss as the server's mac. I dont have any filtering enabled on the VPN server and thus the clients internet traffic is unfiltered. Is there anyway to get the VPN server to use each individual computers mac as it accesses the internet so that I can place all of the computers into the correct filtering group?