When Clarence Parker, the grieving father, receives news of the truth about his son's shooting, he sets his shotgun down by his son's body. When the body is being removed, the shotgun is gone.
Ostensibly the police did not search the shooting victim's body for a knife because they were waiting for the coroner to remove the body, but this would not stop them from performing a search immediately or at least after taking photos. Also, in all likelihood the officers on the scene would have handcuffed the boy after the shooting as a precautionary measure even if he seemed dead.