If your brother's device is rooted: download an app from the Play Store called DisableService. I believe it's free. Open it up, click on the System tab, search for Google Play Services. Once you locate the Google Play services app, tap on it, and you should see a very long list of associated services pop up. Scroll through the list and uncheck pretty much anything starting with the word Cast. Exit out, reboot the device, and voila - the device cannot cast anymore.
This is also very useful if you'd like to prevent accidentally castng porn or something inappropriate on a community/public Chromecast device.
Oh and yeah obviously to re-enable just follow the same steps except recheck all the things, then reboot.