I’m so sorry to hear this.
Keep logs of all the money you’ve paid, all the times they turn up, all the threats.
If your son knows their identity, pass this onto the police.
The police can do something about this if it is reoccurring and if you feel in danger. If they don’t do anything, keep calling. Worst case scenario you should be able to get an order against them for harassment and then if they do threaten you again they will breach their order and (hopefully) be sent to prison.
Drug dealers are pure evil. I’m so sorry that you son was taken advantage of and now you are in this situation.