This has definitely occurred for some students – your son is not alone in this. It sounds like his GPA and/or test scores were below the school’s requirements. Has your son contacted the music admissions office and asked for their assistance in how to pursue this? Sometimes schools will allow students to take one or more remedial classes in the subjects they haven’t done well enough in for the school’s standards. Sometimes this comes with a year-long academic probationary period. If there are any learning differences complicating matters, schools are required to provide testing and other accommodations for students who can show the required documentation.