Your son would have most likely been arrested in several different European countries including the UK. .
Also in several U.S. States who do not have reciprocity when it comes to the CCW / CHL .
California, Connecticut, Illinois, Maryland, Massachusetts, New Jersey, New York.
plus
District of Columbia, New York City.
Finally, his son was lucky he was in a state like Arizona that not only it fully recognize a CCW/CHL from another state,
Arizona is also one of the few states where you can conceal or open carry a handgun without needing a license/permit.