Hello melissafromleonay,
I can see that your'e getting an error message when attempting to accept a Family Sharing invite from your son's device. The following resource offers a variety of different ways to accept the invitation other than via the email. Check them out below.
Set up Family Sharing
Accept or decline an invitation to join a family directly from your device. You'll get an invitation sent to you by email or in a text message. Then you can respond right from the invitation. If you miss the email or text invitation, no problem. You can respond from your device Settings or System Preferences:
- On your iPhone, iPad, or iPod touch, go to Settings > [your name] > Invitations. If you're using iOS 10.2 or earlier, go to Settings > iCloud > Invitations.
- On your Mac, choose Apple menu > System Preferences > iCloud > Manage Family.
To accept an invitation, follow the onscreen instructions. When you join a family, you might be asked to confirm your account information and to opt-in to any features or services your family set up.
If you can't accept the invitation, see if someone else joined a family with your Apple ID or is sharing purchased content from your Apple ID. Remember, you can only join one family at a time, and you can only switch to a different family group once per year.
Thanks for being a part of Apple Support Communities.
Kind Regards.