change the SSID to something else and set it to not broadcast. Unless he breaks out the wireless hacking tools he won't know about it. Just takes a bit of faffing to get your machines set up (unless the router will not broadcast unless the WPS key is pressed).
You still have the "dude, what happened to the internet" question to answer but you can be a bit more coy about it "oh, I started using my phone mate, costing too much".