He has never proved he was.
The evidence he is withholding.
What would you say if a criminal could get away because no one could identify him through assess to his official files?
This argument is an insult to intelligence, a slap in the face of natural justice and if a Usurper can get away with being one by concealing where and when he was actually born, and of whom, the whole political system has broken down.
Like it or not, he doesn’t need to prove it.
If you don’t like that, help work to get state laws on the books that require any candidate for office (any office) prove his citizenship status. That’s the only solution at this point in the game.