This property devolves equally on all the legal heirs of your deceased grandfather.
Since your grandfather died intestate, all his seven children have equal rights in the properties left behind by your grandfather.
The legal heirs of your father's deceased siblings are entitled to their respective father's share in the properties.
Therefore, legally the entire properties are to be divided into seven equal shares and your father is entitled to one such share only.
The option before him is to file a partition suit against all his siblings and the legal heirs of the deceased siblings for partition of properties and separate possession of his legitimate share in the properties.
Consult a local advocate and file a partition suit as suggested which is the legal option if there is no amicable settlement among the disputing siblings.