PPF was account opened in the name of my son when he was a minor ( with me as father&guardian). My son went abroad for studies & after completing studies has started working abroad. My son has attained majority. The PPF account is maturing. How do I close the PPF account?

What documents should I submit? My son is not in India to sign any forms- but he has signed a letter authorizing me to close the account and receive the proceeds. If this letter is to be used , do we need to have his signature authenticated & if so, by whom? Thanks