Sounds like harassment which is a criminal offence and sounds like the it might be worth making a formal complaint against the company, especially as it's not your debt.
This link to the
National Debtline website provides some interesting info on your rights. They've got a helpline 0808 808 4000.
What's your son doing about the debt? It might be worthwhile for him to make an appointment with a debt adviser at CAB or a similar organisation to help him deal with his creditors.