Thank you for trying that Jtckd30,
Surprised that restarting did not help.
As you still have your son's name listed:
You might want to try try again.
Guess this (below) is exactly what you have already done.
On your iPhone, iPad, or iPod touch
- Go to Settings.
- Tap Family. If you don't see Family, tap your name, then tap Family Sharing.
- Tap the name of the family member that you want to remove.
- Tap Remove [your family member's name] from Family.
The only reason it might not work seems to be if your adult son was listed as being under 13 years old... you could check his Apple ID / date of birth is correct. (You would need to contact Apple Support to remove anyone listed as being under 13 years old.)
Your adult son could likely remove himself, (ie: using the linked article above)
Else you could reach out to Apple Support:
There is a "Get Support / start now" link (online chat, phone, eMail etc.)
at the bottom of their webpage (link below):
Official Apple Support
All the best :-)