At some schools, it isn’t hard to graduate in 4 years while changing majors although some major would be an exception to this. Many colleges don’t requite a student to declare their major until the end of sophomore year and students still graduate on time. In fact, those schools often have the highest 4 year graduation rate.
Many schools require a number of core and breadth classes so students often spend the first years fulfilling those then concentrate on requirements for their major during the last two years. It’s likely your son took classes that helped him meet those breadth/core requirements.