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Your son can sue civilly for the threat against him but, realistically, it will cost you more than you're likely to recover if you use a lawyer.
You can't pursue this for your son nor can you represent him or speak for him in court.
He would have to sue them in NJ for the threat since that is where they are located.
You can sue him for the breach of contract civilly yourself and if you were in NY when you gave him the jewelry then you can sue there. If not, then you have to sue them either where you were located when you gave them the jewelry or where they live.
There is a 6 year statute of limitations in NY and in NJ so as long as you are within that time period you can file the lawsuit and won't be "barred".
As you apparently recognize, there is also a criminal complaint possible.
If you decide to use a lawyer then you want one who does Civil Litigation. Since it is breach of an oral contract you can also sue for attorney's fees,
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