Yes, you can put 2 NIC in the PC conected to internet.
The NIC to de DSL w/tcp protocol and obtain a IP automatic
The NIC conected to you son w/tcp and especifi a IP number (ej. 192.168.0.1) your son PC whith a IP number such 192.168.0.2

then dounload WINGATE or PROXY++ (Wingate work 100% OK!)

ok, ok, yust e-mail me and y send a copy of Wingate 3.0

good luck