Suffolk County police arrested 70-year-old Joseph Grucci after they say he fired a shotgun at 20-year-old Christopher Giresi around 3 a.m.
When police arrived, Grucci told them he had a dispute with his grandson and had shot him.
Giresi was rushed to Brookhaven Memorial Hospital Medical Center in East Patchogue, where he underwent surgery. He suffered injuries to his torso and arm and was listed in stable condition.
Grucci is charged with first-degree assault.