Adopted as a child

Hi all

I hope someone can help with a query I have.

My husband was born in Pakistan in 1974. At the age of 4 he was adopted by his uncle ( i.e. his dad's brother) who had british citizenship at the time. and subsequently came to the Uk at the age of 4.

My husband would now like his natural father ( who lives in Pakistan) to come and visit us in the UK. We feel we would have a stronger case if we had some way of showing that he was adopted as a baby and that the "uncle" in pakistan is in fact his father.

There are no legal documents to prove that he was adopted as a baby. Can we use an affidavit to do this?

Any advice on this would be much appreciated.

Uzma