I have a complex query pertaining to the property inheritance

My name is Kuntal Shah. I lost my husband 5 years ago. I have a son with him who is 11 years today. My surname was Javeri before I got remarried. Thereafter my father in law passed away 4 years ago. I got remarried in 2015 and live with my new husband and son now. We haven't legally adopted my son and not changed his surname from Javeri to Shah although he lives with us. Does he have any right on his deceased grandfathers / fathers property ? My ex mother in law refuses to show me the will left by her husband and does not want to give any share from the property or cash to my son who is still a Javeri as per official records. What are my legal rights and power in this case for my son to acquire his share from his original father and grandfathers assets ?