Corpus delicti of the crime

Dear PAO,

My son, together with his friends, went on a drinking spree in a bar and got involved in an altercation with another group. A few hours after he returned home, the police arrested him for an alleged stabbing incident at the place where they were drinking. Apparently, one of the persons involved in the earlier altercation was found dead with a stab wound. According to the police, witnesses pointed out that my son was seen having an altercation with the victim and was seen "hugging" him when the victim fell. However, the knife used was never found. Is not the knife the corpus delicti of the crime?