A 30 year old male was brought for evaluation. The history revealed that the patients 3 year old son died, 5 months back after being hit by a car. At the time of accident, patient was standing nearby and witnessed the accident. For last 5 months, he has been having symptoms of sadness of mood, crying spells, feelings of wohlessness, poor sleep and poor appetite. He has twice thought of killing himself but stopped at the end moment. He has not been attending the office of last 5 months. What is the likely diagnosis: