Please help!! Mentally retarded brother with anger problems

I have a brother who has been diagnosed with borderline intelligence disorder and also personality disorder.
He is 44 yrs of age. Suffering is mainly emotional in nature, tends to speak to self loudly. Mostly these are mental dialogues within him. On observation it was found that these are about the incidents of the past or imaginary situations where he can express his thoughts. Sometimes these are angry mutterings and sometimes happy ones. He is usually oblivious to the fact that there are people around him or others may feel disturbed because of his muttering. It seems he releases anger in such mental dialogues with others, with whom he can’t do it in reality. Keeps brooding about the past.
As a child he has faced a lot of pain and has been bullied too due his low intelligence. He could not complete his education and currently works in a special school. However, this is more of a compassion posting as he doesn’t do much productive work. He is extremely slow in going about the work or daily routine and doesn’t follow strict timelines. While he is Moody and tends to get angry when things don’t go according to his expectation, he is also extremely peaceful at times, typically when he is doing his pooja, or meditation.
Has an extremely sharp memory and remembers details like names and dates very well even after many years. Lazy towards personal hygiene. He keeps his room also messed up. Likes to collect things which are considered trash such as thrown away bus tickets, pamphlets, icecream sticks etc.
Gazes in a way which makes you think, he is looking beyond or sometimes may be considered as staring. While talking, Keeps shaking his head in a peculiar manner.
Extremely regular with his habits. While it is difficult to get him into a habit pattern, but once developed, is extremely regular.
Even though he talks to self and gets angry easily, still his anger subsides easily and can forgive people once they show repentance or explain the reason for their behaviour.
Both my parents have died and he lives with me and my husband. He loves my husband and treats him like a brother. My relations with him are not too cordial as during our childhood I got more opportunities and he didn’t, that has remained a pain point for him. Also, after my mother’s death in 2000, I had to become strict with him and take charge. But over a period of time, the relations have become a little better.
In the last few years one of our relatives had become very close to him and he treated her like an elder sister. However, it seems to have become an emotional dependence and he tends to get angry if he doesn’t get to talk to her regularly. He also scolds her in his mind. Due to various reasons, she has not been able to talk to him for the last few months and he seems very upset about it. Though in front of us he doesn’t say anything but when alone his angry mutterings indicate that. He usually seeks attention from others around him and if he doesn’t get it then he goes off to his mental world and the angry mutterings start.
Four years back I had reached out to a homeopath with his symptoms and was suggested to give him Lycopodium 200. It helped us at that time but only to a limited extent. He has now started to assert himself and speak about his feelings which earlier he used to totally avoid. But the mutterings continue.
While we have come to love and accept him the way he is it would be great if we can help him to become more peaceful.