There are two possibilities. First, if it's a Windows computer, it may have "MAC Randomization" turned on for your network. It may generate a new address automatically every time it gets paused. Do a quick Google search for "Disable MAC Randomization in Windows 10" to get directions for how to check on that and disable it.
The second possibility is that your son is changing the MAC address to get around the pause function. If that's the case, make sure you have the "new device notifications" feature enabled (https://support.google.com/googlenest/answer/10116602?hl=en ) and consider using the operating system's parental control features – something like Microsoft's "Family" or Apple's "Screen Time", for example.
I just wanted to jump in real fast and see if the mac randomization solution worked out for you. Thanks MichaelP for the advice there. Foggerton's solution would also work if you wanted to go that route and have a guest network set up and configured. You would just need to make sure to change the password on the main account so that he only had access to the guest network.
Anyway, if there's more you need here, let me know. Otherwise, it looks like we have a couple of solid solutions. Thanks!
As we haven't had any activity here recently and it looks like we had our resolution, I'm going to go ahead and close the thread. If you have more to add, feel free to start a new discussion. Props to MichaelP for his help here.