It's an odd story.
When I read it the other day It said he was adopted by his grandmother who told him this.
Either she is his biological grandmother and adopted him. So unless she is the queen or camillas mother, it's not true.
Or she is his adoptive mother. Not his grandmother. Why would you adore a child and then take the title of grandmother?
Personally I think it's a story the grandmother made up a story to explain he was put up for adoption because he was special. Not because his parents didnt want him.
He possibly has some mental health issues and believes this. Who knows in later life the grandmother might have believed it too. Especially if she had dementia.
If he is still peddling this story he is either ill and doesnt realise it can be proved he isnt. Or think the royal family will keep ignoring him and never do a dna test leaving him to keep spouting this.
Exh nana had dementia and truly believed she met her husband on an Hawaiian island, where she was a hula girl and he came ashore with navy and they instantly fell in love.
It was a story she used to tell the grandkids when they were younger. They met at the irish club in Leeds. But in her last years she used to talk about it all the time. Describe Hawaii and her time as a hula girl.
Maybe the grandma just told people this to impress them and he believes Lots of people have dined out in 'I am Anastasia' or 'I am a 4th cousin of the queen' when its jot true.
I imagine that was a lot easier 60 years ago.