Question about adoption

I am from Tamilnadu. My grandfather adopted my father at the age of 6. however my grandfather did not write any adoption document at that time. However he wrote the adoption document when my father reached age of 21. My grand father mentioned in the document that they adopted my father at the age of 6 and they missed to write a adoption document at that time and now they write the adoption document. Later there were a case filed against my father by relatives of my grand father that my father can not claim rights over properties of my grand father hence lower court also accepted the argument and cancelled the adoption of my father and also mentioned my father do not have right over properties of my grand father. However my father appealed in district court and they given decree partially favour to my father however the court raised substantial question of law about adoption hence they revised cancellation of adoption passed by lower court and hold the adoption of my father. Now the opposition party appealed to high court that the appellant court wrongly revised the decree passed by lower court. Could you please let us know my father can claim rights over properties of my grand father and adoption is correct or not?