I am sure he would benefit from a talented, compassionate therapist; you may want to suggest that during one of his better periods and frame it in the context of your wanting to see him in less pain and feeling like you're powerless to help him.
Sometimes just directly asking him to tell you what he misses most about his father is a good, honest introduction to letting him talk for as long as he wants. I know you want to ease his pain and that his behavior may frighten you, but all you and his friends and loved ones can do is name his pain and let him know you are there day and night for whatever he needs. You are a kind friend for wanting to be there for him during this horrible time.