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Has he ever been arrested for any other criminal offence?
Has he ever been given a reprimand, warning or referral order?
Breach of the Peace is not a crime of moral turpitude.
I will get the legal reference for you shortly.
I assume that would mean he would not need to apply for a visa
He does not need to apply for a visa. I have double checked the legislation and breach of the peace does not fall into the category of moral turpitude.
Am I also right in thinking that as a juvenile he would be exempt from admissibility anyway
Here is a very helpful article confirming the position: http://www.usvisalawyers.co.uk/article13.htm
They have a general discretion to exclude offences involving moral turpitude committed before the age of 18 years. But, of course, this would not apply to your son anyway as breach of the peace is not such a crime.
As far as I can see his arrest will not present a problem and he need not apply for a visa.