There are a number of things you can do.  First, you should make a police complaint against the boy for assault and battery.  In legal language, threatening harm to another with the intention of doing it, is assault, and then following through with the violence, is the battery.  These are crimes, and if the other boy is 18+ he can be tried as an adult (and under some circumstances, even if he is under 18).  Also, you can file a civil complaint against the boy  if your son was hurt, and for his pain and suffering as well.  If the attack was on school property you can file a civil complaint against the school as well, because they were told of the threat and --apparently-- did nothing about it.  Note: you will need a lawyer to see you through these legal steps.