Hi there! I also began stuttering at an early age and can sympathize with what your 6 year old is going through, especially in school. The most common app to help stutters would be Stamurai, although there are others. With that being said, I think looking at stuttering techniques online or on YouTube would be the next best thing. I know practicing at home helped immensely, as it provides a private space to speak without embarrassment. There are also speech books out there that focus on sounding out vowels, consonants, and general phrases to get you by in everyday life. If he keeps working on speech techniques learned from the app, online, or in the books, I guarantee you he will only improve. When I was in school, I could barely speak at times, especially in the early stages. Now I've graduated high school and can interact with people on a daily basis, even if there are days worse than others. I hope this helps and good luck with everything.