2 days my son starting stuttering out of the blue. It this how stuttering happens?

2 days my son starting stuttering out of the blue. It this how stuttering happens? When it started heooked at me kinda scared and said - Daddy, I can't talk. I feel really bad because I don't know how to help him. I made a doctors appointment but I'm wondering how it just started like that. He's had trouble pronouncing words and sometimes talks way too fast but never stutter. Anything I can do to help him?