The Default Gateway heading, filled with X's is the one we're looking for.
Open a web browser --> type in that number, including the dots.
It should now prompt you to log in. The password should be on a sticker on the router.
Once logged in navigate to the advanced settings page, unfortunately I cant be any more specific as each router has different menus. --> Find the MAC filter function.
Now you need your sons MAC address, so you'll need to borrow his phone.
Navigate on his phone to Settings --> Wireless and Networks --> Wi-Fi Settings --> Click menu button and select "Advanced" --> The MAC address should be listed here.
Now that you have the MAC address, you can add it to the filter list in the routers settings. You can disable and re-able access in about 10 seconds flat, if you leave the routers IP in your browser history (or even bookmark it).