At any stage of the criminal proceedings, including appeal or other post-conviction proceedings, the court concerned may for good cause assign a substitute attorney. The substitute attorney shall have the same functions with respect to the indigent person as the attorney for whom he is substituted. If the substitute attorney is not employed in the office of the public defender or in a nonprofit legal aid agency serving under this article, the court shall prescribe reasonable compensation for him and approve the expenses necessarily incurred by him in the defense of the indigent person, as provided in Code Section 17-12-5.