First wait until both of you are calm and somewhat stationary.
Ask him about the event in question.
When he tells you what happened, calmly repeat what you heard him say.
Do you best to not freak out. Just continue the cycle above, repeating and asking for more, at least until you've heard enough for now.
If he has told you something difficult to hear, again you have to do your best to not freak out.
Thank him for his honesty, and carry on calmly.
most people want to tell the truth.
But they first want to know you're going to handle it well.