My grandson did not utter a word until he was four. We learned and taught him to sign. It was not a simple path, but there is hope. My grandson is 14 now and will not hush. Every situation is different, so I will pray that your grandson finds his voice soon.
We never stopped speaking to him directly; we read to him often and exposed him to many different types of activities. I mention this because when he finally began to talk, he left us all speechless because he knew how to read. We would pick up one of his books, and he would tell us all about the story inside. So though nonverbal, he was still learning at a high rate of speed.
You aren't alone.
God bless you and your family.