Just trying to help him with some lateral thinking.
In the meantime, as the cruiser and his TA are getting nowhere at the moment, surely plenty of fellow countrymen will already have had to deal with this generic (Carnival-wide) issue - he needs advice from someone who has already faced the issue.
I maintain some Cunard USA passengers will have done so, and they are a helpful and vocal lot over there on the Cunard site.
If the cruiser finds out how it is done with USA Cunard Southampton passengers then he only needs to ask for confirmation from P&O that the same procedure applies to him, which it surely will - how can P&O and Cunard not have matching rules re proof of vaccinations requirements for USA passengers embarking at Southampton?