What you describe makes me wonder if he is experiencing posttraumatic stress disorder. Do you know if he has experienced any type of trauma possibly? Or it could be depression, although the part he says about something happening makes me think more PTSD.
I know it is difficult when he won't open up and when he won't talk to you. That is common with certain mental disorders. best thing you can do is just try to be supportive, keep calling him, but don't push him, and let him know you care about him, and are supportive of him no matter what. Maybe with time he will open up, but he will do it on his own time. show you still care and still want to be part of his life, but be patient. If you push too hard he will only push back harder.
I hope that helps. Let me know if I can be of further help. If you are satisfied with my response please click Accept.