How can I stop my child from using his iPhone hot spot to stream stuff I don't want him accessing on a laptop?

How can I stop my child from using his iPhone hot spot to stream stuff I don't want him accessing on a Chromebook?

I can easily block his Chromebook (and other devices) from my home WiFi network, using the Xfinity family app. But then he switches to his iPhone's hot spot and goes browsing on the Internet using his Chromebook (which he is supposedly doing schoolwork on but I know that's not always the case). That is how he bypasses our family WiFi, and this enables him to waste his time on SuperCell and other free online video games and probably worse. I don't know how to stop him from doing this.