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 a man dies and leaves behind a son and a grandson, the grandson is prevented from inheritance due to the existence of the son. Both the son and the grandson are male inheritors from father's side but the son has priority over the grandson as the Prophet, sallallaahu ‘alayhi wa sallam said: "Give the prescribed share of inheritance to those who are due it and the remainder [of it] is for the nearest male relative." [Al-Bukhari and Muslim] However, it is desirable for the heirs to give to this grandson whatever they can in order to keep ties and good relations with kin, as Allaah says (what means): {And when [other] relatives and orphans and the needy are present at the [time of] division, then provide for them [something] out of it [i.e., the estate] and speak to them words of appropriate kindness.} [Quran 4:8]
Allaah Knows best.