IQ diagnosed as 86. For anger control?

My nephew who is 9 years old has been diagnosed with IQ 86 last week. After the boy's mother suspected very weak in studies, cannot remember dob, what he studied a day before, alphabets , extreme anger that threatens to commit suicide. Report yet to receive. However we are quite depressed with this. He is studying in cbse school in chennai where the pressure is high. Basically i would like to hear from experts here 1. What is the root cause of this? 2. What we should do at home/ school to improve his state 3. What kind of mental exercise will help to increase? 4. Any do s and dont s 5. Anything related to environment or food? 6. Any suggestion for anger control? I really hope doctors or experts will help me and my family to move forward with a better solution to shape my nephew /son to a better future. Thank you Worried aunt Sindhu