Although your son did not have the knife, he was at the scene of the attempted robbery with a person who had the knife.  Therefore, your son would be an accomplice to the attempted robbery.  An overt act in furtherance of the crime is sufficient for one to be charged with attempt.  There is also the possibility that your son and the person with the knife engaged in a conspiracy to commit robbery and although your son did not carry the knife, he would still be liable for conspiracy. With regard to accomplice mentioned above, some jurisdictions use the term accessory.  An accessory after the fact would be someone who asisted the perpetrator (the one with the knife) to escape, etc.  An accessory before the fact would be someone who assisted with acts preceding the commission of the crime.