A "disposition" setting typically means that the defendant has already had a status conference or first setting, and that at the next setting, the case either must plea or set for trial (or hearing if it's a Motion to Revoke Probation or a Motion to Adjudicate and Revoke). It's basically a designation by the court that the case is not new, and should be dealt with in the near future.
Generally, disposition in a criminal case such as this instance would mean that at that setting, the case will either be settled by an agreement approved by the judge or the case will be set for a hearing at which the judge will determine if there has been violations, and if so, what the outcome should be.
If he has not been appointed an attorney already due to his being in custody, he will likely be appointed counsel on or before this "disposition" hearing. That attorney should visit him in jail or at the courthouse that day and discuss his options. If it is a Motion To Revoke his probation, the State may make a plea offer. But he will have the right to contest the allegations of the probation violation(s) at a hearing before the Judge if he chooses. His attorney will be able to help him make that decision.
Although my intent in answering this question is to aid you in the legal process, my answer does not establish an attorney-client relationship in any way. You should seek the advice and counsel of a qualified attorney in your community to evaluate your legal needs and to advise you. No Attorney-Client Relationship is created without the specific intent of both parties.