What happens when one of the beneficiaries dies before the decedant and the will was not changed?

My son's great grandmother died in April. His grandmother and father died before this (2 years ago). When the great grandmother died her will had listed the grandmother as executor of the will and half of everything went to her as well the other half was to be split with her brother. My son was not born when the will was written and his father was not mentioned in the will either. It was a straight forward will. I just recieved papers today saying that my son is a beneficiary but the will has no mention of him. Does he recieve his grandmothers share?