Keep contacting and communicating with the HR and after couple of emails mark cc to the management. When marking to the management make sure to acknowledge the email of the HR wherein he had promised that the relieving letter will be sent in 2 weeks time to bind him. You may use that email as your relieving letter just in case it is needed in the meantime. You may even indicate that you will be using his email ack as the relieving letter so long as you do not receive the official copy. Keep your letter of notice and other docs intact for any future use.
Thanks and Regards
14th February 2019 From India, Hyderabad