How can I link my son's Apple Watch with a cellular plan to one of my old iPhones?

If you are setting up an Apple Watch for your kids, it has to be a parental iPhone, and the cellular provider has to support this feature.

Set up Apple Watch for a family member - Apple Support

Make sure that the watch is compatible with your old phone. Apple Watch and iPhone compatibility - Apple Support (CA)

Go to the watch app and follow the instructions in the app.

Hope that helps.

You're welcome 🙂