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.