In my experience, you can't do anything while the child is still overwrought. You can only find out about it later.
If the child isn't too overcome, you can try;
a. Happy face strategy - "I can't talk to you until you stop crying"
b. Ask him to draw what happened - it probably won't be detailed, but later he may explain it to you.
c. Yes/No Prompting - "Where did it happen inside or outside?", Did someone hit you?
If he can't answer, speak clearly and give him two choices only. Ask him to touch one of your hands for yes and the other for no. Sometimes they feel more comfortable with that.
