My father was builder. he sold the flat to a customer and done agreement to sell by giving the posse

My father was builder. he sold the flat to a customer and done agreement to sell by giving the possession in year 2002. But the customer did not done Execution of sell deed and died in 2006. He's son is living in the Flat now and ignoring to do the Registry of flat. Now my father also died in year 2019. I have given notices for the dues and execution of sell deed but he didnt reply and ignored. I have to cancel the agreement and want to take the possession back from. please tell me suggession.