Watch Familly sharing
Can my son pair his Apple Watch with his iPhone as well as my iPhone under family sharing?
Apple Watch Series 4
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No. Multiple watches can be paIred to a single iPhone but not the other way around. Why would you want to that? Family Setup in watchOS 7 and later lets family members who don't have their own iPhone enjoy the features and benefits of an Apple Watch. If your son has an iPhone, why would you want to use Family setup?
Set up Apple Watch for a family member - Apple Support