The following answer only applies for graduate schools in the United States that are members of the Council of Graduate Schools, and have signed on to the CGS Resolution Regarding Graduate Scholars, Fellows, Trainees and Assistants. (Here is a list of these institutions.)
Furthermore, this agreement applies to offers of financial aid, not admission without aid. (Although many schools adopt a similar policy for offers of admission without aid.)
According to the Resolution:
In those instances in which a student accepts an offer before April 15, and subsequently desires to withdraw that acceptance, the student may submit in writing a resignation of the appointment at any time through April 15. However, an acceptance given or left in force after April 15 commits the student not to accept another offer without first obtaining a written release from the institution to which a commitment has been made.
So the answer is: he can withdraw (in writing) his acceptance of financial assistance offered by School A before April 15 (if School A has signed on to the Resolution).
However, for general reference: you should really refrain from accepting an offer until you hear back from all the schools you are seriously considering.