Hi Mary-anne224,
Welcome to the Apple Community.
Re: Any thoughts?
You might want to check the calls have not somehow become accidentally "blocked"
If so, you could "unblock" your son's access
Block phone numbers, contacts, and emails on your iPhone, iPad, or iPod touch - Apple Support
Block calls and messages from certain people – Apple Support
Detect and block spam phone calls - Apple Support
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There may be a setting in use that is limiting calls from people who are not listed in your contacts:
Discussion thread about access to that setting:
how to block calls that are not in my con… - Apple Community
You might want to check that your son is properly listed in your "Contacts": app.
Manage and delete contacts on your iPhone, iPad, or iPod touch - Apple Support
All the best :-)