Grandfathers will

My grand father had prepared a will for which he has given the property to his two sons and a daughter out of three sons and a daughter in the year 2000 and he expired in 2005. The will is registered will with two nominees out of which one nominee has expired and second is a advocate by profession.there is a doctor also who has given medically fit certificate for my grand father to prepare a will with his signature on the will in 2000. The thir son to whom the property was not given expired in 1999 and due to his inlaws interference his family left our house and started staying in a rented property since 2000 and now his son is coming and asking for his share in the property. Note:while leaving the house they took all the money his dad had left behind.they were not forced to get out but they left on their own with all the money.the grandson who is asking for he share is a drunken maniac.his condition to survive with his family is bad and now asking for share. On what grounds he can challenge.or will he be able to challenge this will. For precautionary measures what should we do. Regards, Raj