The thing is, he is only looking at his parents birthplace, and not going back any further, which is disingenuous.
Where were his grandparents from? Where were their parents from?
We are all a complete mix of ethnicities, so trying to identify as one race only, really makes no sense.
I was born in England. I have lived in Scotland for 20 years. I feel "a bit" Scottish, as this is where my home is, and where I have built a life. I did a My Heritage test, and in fact, I am 30% Scandinavian (plus a few other things). I have never been to that part of the world, and feel no affinity to it whatsoever.
My children's father identifies as Irish, simply because his parents were both born there. My children have been known to tell people they are Irish. But I think a lot of this stems from their Grandparents who just never stopped banging on about Ireland, even though they both left in their teens. Constantly buying the grandchildren Irish sports tops, always supporting Ireland in any sports. It used to annoy me, I felt like saying "You've lived in England for 50 years, when will you choose to assimilate?"
In my experience, the Welsh, Irish and Scottish like to protect their identities fiercely, whereas English people don't seem to get so worked up about it.
But the whole thing is silly, because if your Partner did a My Heritage blood test, he would see that he is made up of several nationalities. We all are.
If he loves Wales so much, and is disgusted to be called English, maybe he should bugger off to Wales and stay there.