Grandmother false will

Hi, My grandmother has a property which was her parents home but since she was the only child. She got it. She had 3sons and a daughter. Her husband died in 1998 and she died in 2001. His elder son died in 1996 and second son died in 2007. The youngest has created a false will of his grandmother, which says the whole property belongs to his son bcoz she wanted it in the year 1998 after her husband died. Bt its false. Is there any possible way he can do it? If yes, what are the ways we can prove him wrong