You have a constitutional right to represent yourself (the Faretta right), although the judge will often appoint "standby counsel" if you realize how utterly stupid that is to do. You can do this even if you're literally nuts like the Long Island Railroad shooter. Unless you are actually held incompetent to stand trial at all, you are (generally) presumed to be able to make informed choices about your own legal representation.a
There are few exceptions to this and generally include extreme disruption of the proceedings or repeatedly abusing the right by continually firing your lawyers and delaying the proceedings, or by being mentally incompetent, which would probably be the sole relevant reason for denying the right in this case unless he starts throwing autistic tantrums in court. The fact that he is literally on disability for mental reasons is at least somewhat probative, and enough for the court at least to look into it, but generally the bar is low, as witness the LIRR shooter, Colin Ferguson.
Naturally it's a really bad idea to do this because it is ridiculously awkward to be cross examining witness after witness who personally witnessed you committing the crime(s), like Colin Ferguson. The witnesses were actually getting pissed with him, basically saying "Yes I saw YOU SHOOT THOSE PEOPLE YOU SON OF A BITCH" (not from the actual transcript).
You're also fighting against someone who does this shit every day, and you probably don't know the stylized formal combat that is cross examination. A skilled prosecutor will break up your flow by throwing around objections, especially if you don't actually know the rules well enough to respond to an objection or evidentiary argument on the fly.