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Can you post the exact model number of the belkin router? Will allow us to look up it's capabilities.
Does the router have wired LAN ports (often there will be 4) as well as a WAN ethernet port (socket)? If so you can keep wired connections to the two PCs. I suggest that now you have a router you can ditch the shared connection from your desktop to your son's desktop. You'd connect your sons desktop directly to the router. That way you no longer need your PC running if your son want's to use the internet, and the slight load of the internet connection sharing is removed from your PC. The router now takes all the load.
To simplify the way everything is wired up at the moment you'd put the router near your office desktop.
Wired connections:
STB > router
Router LAN #1 > office desktop
Router LAN #2 > Son's desktop
Wireless from router to new lappy.