I would use the behavioral treatment outlined by Busse (2013).

  1. Give numerous opportunities for the child to speak. Do not ignore them or speak for them when they are choosing not to speak. 
  2. Ask questions throughout the day, leaving significant time for them to respond.
  3. Use questions and rely on responses to make decisions throughout the day. Use opportunities where the child speaking will result in high preference items and not speaking will result in low preference items.
  4. Overly praise and validate all verbal communication and speech.