Your employer has to complete certain documents re. your employment, and they have to prove they have taken the necessary step to ensure you are legally entitled to work in the UK, and can ask for you to provide said documents (and / or others)
He’s doing it wrong though, as he’s actually supposed to see the document, and then sign the copy to attest to the fact that the original document was seen.
They can actually ask the same thing of longer serving employees too, though many don’t bother.
Do check though that your employer complies with the Data Protection laws regarding the security of employee records.