Purchasing of a joint property with only one legal heir alive

I am planning to buy a flat which is jointly owned by a son & his mother. Mother has expired long time back & she has not left any will. Even the father of son / husband of wife has expired few months back. Son is claiming that he the only successor of his parents & do not any brother/sisters. But I am not sure about his claim. As per son the property can be sold & registered in my name with the help of death certificate of his mother. Some friends of mine have advised not to go for deal without a proper will (which is not available) or letter of administration / succession certificate (which is a time consuming process). Now, I am confused on following points: Whether I should ask for letter of administration OR succession certificate from seller OR BOTH. Does letter of administration applicable in cases where will is not available. Is it possible to close this deal through gift / relinquishment deed / legal heir certificate? Please advise whether I should go for this deal or not.