Does he have a private teacher? And/or are his school teachers experienced with students going to UMD? Usually experienced teachers can let him/you know if he has the similar skills as other students who have been accepted at certain schools.
If he does not have experienced teachers, he may want to reach out to a faculty member at UMD to do a lesson (be prepared to pay for the time) to determine his chances. An experience faculty member would know within one lesson the level of school.
Using experienced teachers/faculty is how most parents/students develop an appropriate list of schools.
If you have already done this and are just having a hard time “believing” it, most teachers really do know. So you need to trust them to make a good list of schools. If you do this and have 6ish good schools, he should get some acceptances.
Good luck!