The very same thing happened to my son when he was that age. It was sudden and pretty upsetting to have a child that could talk suddenly not be able to get his words out. And you're right, it's heartbreaking to watch their frustration. Mine was already set up for speech therapy through his school because he had a speech delay when he was younger, so I immediately got in touch with the therapist to help him. We ended up switching to speech therapy outside of his school because they had more experience with it. He's 14 now and still stutters pretty severely depending on the situation and who he is talking to. Speech therapy has helped give him "tools" to work through times that he is stuttering. The most important thing we do is support him and teach him to not be ashamed of it. He's always been very good about explaining his stutter to people when he's having trouble speaking. Adults are always understanding, and most kids are fine with it. Hang in there. I know how hard and upsetting it is when this hits you out of nowhere.