Meanwhile, if they seized his phone and tried to unlock it without a warrant signed by a judge, that is a violation of his rights as per the fourth amendment of the Constitution of the United States. However, in the US presently, such violations are routine by law enforcement.
Some agencies insist suspects do not have access to those rights if certain conditions apply, such as being within one hundred miles of a US border (such as a coast), or being on a terror watchlist.