Thank you for your question
My name is Clare
I shall do my best to help you but I need some further information first.
Which part of the UK does your son live in?
Were the police called?
Did the others leave?
have they threatened to return
How old is your son?
You cannot make this applictaion - it would only be of use if they were "squatting" in the property - if they had entered and remained there which they have not.
Your son could potentially try for an Injunction using the Protection from Harassment Act 1997 - although more correctly it is his friend who could should apply.
IN general terms however it is for the police to protect them.
Please ask if you need further details
No I am afraid it is just the Protection form Harassment Act
NO I am afraid not