Hello, I’m 18 years old but i also developed a stutter around the same age. Patience. It’s gonna take years to see any improvement. You sound like you’re doing an excellent job being a parent and putting him in speech therapy. I did therapy as a child, but I eventually grew out of it and stopped before I was 6. I had a moderate to severe stutter when I was 3 years old until around 7-9. After that it began to improve and now it is very mild to none. Definitely keep going to speech therapy, once a week is just fine especially for a kid. One important thing is talk to him like you would anyone else. Don’t ask him easy questions, challenge his speech but be super patient and allow him to answer all the way. But also don’t push him to far to the point of getting upset. I wish I had done more therapy, but it takes time to see improvement. You’re doing a great job, you got this.