@Greybird 

Under the Wireless settings you'll see CHANNEL. You have to be on a Browser and logged into the router, and then use Wireless on the Advanced tab. That is where you change the Channel the SSID will use.

If on Windows, you can get a program that will display all the various Channels in use. Here is a link to a free program that you can use (on a wireless router connection what is required), https://www.acrylicwifi.com/en/

If you don't have a wireless device, only wired, the next best thing would be to see if your sons headsets specifications say what channel it is using on 2.4Ghz. Doesn't have it specificied, I'd suggest changing the channel and seeing if that helps? Pretty sure there would be a channel that works for you.

@Greybird Thanks.  The tester helped narrow down the options.  I was able to reduce the interference by setting a channel and ruducing power slightly, but it didn't eliminate it entirely.

@Greybird 

You can also adjust the WIFI bandwidth as that may help. 

Also try turning the router so it faces a different direction as the signal is not a perfect circle. Changes as small as 20 degrees can make a difference. Also if you can move the antennas can change the signal pattern.

@ArcherC8 Are the individual antennas directional, and does lowering them disable them?  The side of the router that faces my son doesn't include any actual devices (past him is  a section of stairwell, then part of the kitchen.)

@Greybird 

to some extent, all antennas are directional. A single antenna for the bandwidth is almost a circle but signal interference from other devices and objects it bounces off of affect the signal. Multi antenna for a bandwidth can focus the signal. Many routers use beamforming or direction to try and get the strongest signal to the devices.