The property is in the name of the grandmother. Your father has to issue a notice to your grandmother requesting for the partition of the property and demand for his share in the property and follow it up with a partition suit. (I am afraid that grandsons are not eligible for ancestral property partition during their father's lifetime and they can only claim their entitled share from the father after he gets that property from the partition.
Property matters in Hindu law appear to be simple and also more complicated. Contact a local advocate for proper guidance so that he can handle the case right from issue of notice and dragging the court case for another decade in future, if GM is not willing, and misleading the successors during deaths etc., during the next ten years.