Signed on blank stamp paper

Hello sir/ mam, My garndfather given his son signature on blank 20rs stamp paper.he told that he will take care of my grandpa and gradma.but he lied.now my grandpa is very serious and admitted in hospital My mother only taking care of him. Now my grandpa wants to give will power of his property to my mother. Can he(mother's brother can misuse that stamp paper). Grandpa not able to come register office to do will power.can we do while he s n bed?pls help me