Or not... Anything with a special needs student requires more consideration. Discuss with the student, and the parents. If the student has an IEP, discuss it with the special ed department. If the student is made aware of how important he is to the ensemble by playing tuba, he may actually be more inclined to stay. After discussing with all parties concerned, then make a decision. And if the decision is that it is not best for the student to play tuba, then make your adjustments.