Actually, looks like he cannot visit. UK citizens who don't have granfathered rights are 3rd country citizens. And he doesn't fit the allowed exemptions.
Using the info provided, Travelcheck gives this:
Travelcheck
Summary
Please review your answers
Would you like information on entering Switzerland or another country?
Entering Switzerland (incl. transit)
Which of the following statements apply to you?
Other
Which country are you entering from?
United Kingdom
Which of the following statements apply to you?
I have not been vaccinated against COVID-19.
What is the reason for entry?
Please also note the info boxes ().
Tourism / other trips
Result:
Travelcheck
Entry is not currently possible. You want to enter Switzerland from a risk country but you do not meet the requirements. For this reason, entry is not possible.
Minors can always enter the country accompanied by adults who are also entitled to enter.
The entry restrictions are regularly reviewed and are changing all the time. However, we cannot give you any details about specific changes.
For further information, please see the website of the State Secretariat for Migration SEM .
So it seems he cannot visit, because he does not fit the official definition of Visiting Close Family:
Visiting close family (spouse, registered partner or children under 18) who are resident in Switzerland Exercising a visitation right (under civil law) for children and their accompanying person; this includes the child's entry to Switzerland Birth, wedding, serious illness of grandparents, parents, siblings, children or grandchildren resident in Switzerland Visits between couples or persons in other close relationships who are not married or in a civil partnership and do not have shared children (long-term relationship)
He needs to get vaccinated!
Kind regards
Ian