A mentally slow person CAN go to prison. Many do. Communicate with his public defender. If the public defender is not serving him well then get the family together to raise money for an attorney.
The above information does not establish an attorney client relationship nor is it meant to provide legal advice.
I think the first questions you need to answer are( is he mentally capable of committing the crime. You state that your son has issues. Are these issues the result of low IQ, i.e., mentally retarded; or is this the case of an axis I diagnosis, i.e., bi-polar, anxiety, depression, etc.)
If your son has such sever issues that he cannot help in his own defense, then your son's attorney may want to look into filing a suggestion of incompetency along with a written motion for not guilty by reason of insanity. The end result is that your son will have a forensic work-up performed by by a qualified court ordered doctor. Depending on the result, your son could be placed in a facility such as Summit, which is a locked down mental facility that your son could be placed for up to 6 months.