These kinds of incidents are common when people get random calls and threats. What you can do in this situation is, first, do not panic as it involves your son, stay focused a calm. Secondly, deal with the situation confidently and fearlessly. Don’t show that you’re afraid of them, don’t let your son go out of your sight for some time. Take some precautions like involve in your son’s friend circle, and letting your close ones know about the situation. Ask your son to be alert. 
Ask the threateners what they want and why are they doing it. I know they are not going to tell you so easily, but you gotta be rigid with them. Warn them that you will do a police complaint and hence they will be arrested. If the situation still doesn’t change, do not hesitate to pay a visit to your nearest police station.