Insanity defense requires three-way test

DEAR PAO,

My son, who is suffering from a mental illness, hacked his cousin to death. Before the incident, my son attempted to commit suicide on many occasions. Also, he is suffering from insomnia and cannot recall his actions, even the most recent ones. Most of us observe that when we have a conversation with him, he is caught blank staring and would not respond. We plan to testify and raise this issue to support his status as an imbecile and exempt him from criminal liability. Is there any guideline set by the Court in availing the insanity defense?