Does the police have the right to search my sons backpack, and arrest him for the contents?

My son and his friend were walking on the train tracks as a shortcut. He is 15yo and his friend 14. About a block away on the path, they saw someone walking towards them, and then realized it was a police officer. They said he never told them to stop. They both reversed course and started walking back the opposite way towards our home. They jumped the train track fence, and by the time they made it over, there were police on the street who stopped, searched, and arrested them both. When they opened my son's backpack, he had brand new spray paint cans inside, and his friend had paint pens.He is being charged for vandalism(Sect.594 a/b1), possession of graffiti tools (Sect.594.2a), and resist and obstructing of an officer(Sect148a/1.