1. Yes, your American citizen son can become Karta of a HUF.
2. For change of Khatha, if the property is situated within the BBMP limits, a prescribed form has to be obtained from BBMP and the details have to be filled up and it has to be submitted in the jurisdictional BBMP office enclosing therewith notarised copies of Death Certificate, Property Documents, Family Tree or Legal heirship certificate, Encumbrance Certificate, Latest Tax Paid receipt.
3. To change the Khata, the above documents can also be uploaded online in Bangalore One Centres spread throughout Bengaluru.
U R Welcome.