Remote access to LAN thru Proxy server

My boss wants to access one of our LANs from his home for file sharing. We have 5 LANs connected by ISDN and DSL that get Internet access from a central Backoffice (Proxy) server. He has 512k DSL at home and I am wondering if I can set him up to bypass the proxy server without compromising existing security. Will he need a static IP address?