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 is His slave and Messenger.
If the heirs of the deceased are only his son and his father and he did not leave any other heir, then his father gets one-sixth as his legal share due to the existence of a direct heir (his son). Allaah says (what means): {And for one’s parents, to each one of them is a sixth of his estate if he left children.} [Quran 4:11]
The remainder after the share of the father will be 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); as the Prophet said, "Give the prescribed share of inheritance to those who are eligible for it and the remainder [of it] is for the nearest male blood relative.” [Al-Bukhari and Muslim]
Therefore, the inheritance should be divided into six shares; the father gets one-sixth, which is one share, and the son gets the five remaining shares.
Allaah knows best.