If he has been adjudicated, or determined by a court, to be mentally incompetent and to require a legal guardian or conservator, and one has been appointed, then he may not be questioned without that person present, the same way the police are limited in how they could question a minor child without a parent or legal guardian present. Furthermore, if he was or is determined to be mentally incompetent, then if he was questioned, his confession(s) or statements can possibly be thrown out for his lack of capacity. But if he is mentally competent, even if not as developed or advanced as he should be, or even if he may not be competent, but has not been found to be incompentent (a person is competent until and unless a court finds him otherwise), then he can be questioned like any other adult.You need to get an attorney who will look into having him declared incompetent (if he is). If his public defender won't or doesn't care, try contacting local law schools--many have clinics where law students provide assistance for challenged or at-risk or vulnerable persons under the supervision of professors. Call advocacy groups for the mentally challenged--some may have, or at least can refer you to, attorneys who provide assistance in cases like this. And try contacting the Community Health Law project--they help some developmentally disabled or challenged persons. Here is theirnational website: http://www.chlp.org