You will need the recent (not more than 6 months old) assessment report of psychologist's or psychiatrist's diagnosis along with recommendations listed, mainly stating that he would need the Adult prompter, extra time and a few little breaks inbetween.
Assessment should be done in a corner of the exam hall at a distance from other students or in a different room, to minimize distractions for the special needs students and the rest. The adult prompter should be allowed to ask simple "Wh" (what / where / when / how questions) to get the answers from the students if and when needed, due to their speech delay.
Do opt only for external examination (in other words, opt for only the common / general question paper that the entire nation takes). Don't opt for Internal examination - as these question papers are much harder compared to the standard papers, for some reason. The only concession would be that the student will get to take the exam in their own schools.
Do make sure that the Adult prompter spends some time beforehand & she does know about the student / how to get the best from him, prompting procedures, effective reinforcement technique etc, so that the student will be able to respond well during exam too.
(If any school denies Adult prompter facility or for any kind of issue, please refer to the websites listed in ACHIEVEMENTS chapter).