All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
If the deceased did not leave any other heirs except his son and two daughters of his son, then the entire inheritance is for his son by Ta’seeb (by virtue of having a paternal relation with the deceased and not having an allotted share, so he gets what is left after the allotted shares have been distributed), and his two granddaughters do not get anything from the inheritance as they are prevented by the existence of the son.
This is in case the father of the two granddaughters died before his father; however, if he died after his father, then the inheritance is divided into two shares: each son gets one share, and the share of the deceased son, who died after his father had died, should be divided among his heirs, among whom are his two daughters.
Finally, it should be noted that inheritance is a very complex matter, so a mere Fatwa which is an answer issued according to the question is not enough. Rather, the matter should be taken to an Islamic court to look into the case and investigate, as it might be that there is an heir who would not be known except after investigation. Also, there might be a will, debts, or other dues that are not known to the heirs. It is known that these rights take priority to the right of the heirs in the inheritance. Therefore, the inheritance should not be divided without resorting to an Islamic court, if there is any, in order to fulfill the interests of the living as well as the dead.
Allaah Knows best.