Next of kin is legally defined as the nearest blood relative and that would be you.
The law does not recognise partners who are not married. They are simply a couple who are living together.
He may be noted down as a person to contact in the event of anything happen but that’s a different thing than being next of kin. Unless he has power-of-attorney, he has no say so in her welfare treatment or anything else.
If the worst comes to the worst, then your say is the say and what you say goes. What you have been told by the medical staff is correct.
If the partner seems to think different, he needs to take legal advice because he is wrong. That is of course unless he has power-of-attorney which changes things completely.
Thank you for letting me assist you with your legal question. I am glad that I was able to help.
I am not certain whether that answers the question for you or not, but I am happy to answer any specific points arising from this.
It will be my pleasure to help you again either further with this or any future questions you have