Reason why Holden might be failing out of school and wandering New York City

Post traumatic stress disorder (PTSD)

Post-traumatic stress disorder symptoms may start within three months of a traumatic event, but sometimes symptoms may not appear until years after the event. These symptoms cause significant problems in social or work situations and in relationships.

 They are generally grouped into four types: intrusive memories, avoidance, negative changes in thinking and mood, or changes in emotional reactions.

• Trying to avoid thinking or talking about the traumatic event
• Avoiding places, activities or people that remind you of the traumatic event
• Negative feelings about yourself or other people
• Inability to experience positive emotions
• Feeling emotionally numb
• Trouble concentrating
• Trouble sleeping

Depression

• not going out anymore
• relying on alcohol and sedatives
• feeling guilty
• feeling miserable
• tired all the time
• sleep problems
• loss or change of appetite

The five stages of grieving and loss

1. Denial & Isolation
2. Anger
3. Bargaining
4. Depression
5. Acceptance

The impact of death on a family 

Bereaved Children Speak


I think Holden is going through the stages of grieving and loss, Kids & teens who have experienced  a death are most likely to experience depression, struggle at school, use drugs and alcohol, and thats pretty much whats happening to Holden.
Doctors will treat him as a normal kid who lost a family member, he wouldn't be taking medication, but he would be going with a phycologist that could help him get over the impact of death.