Im 17 and have a stutter myself. I'm currently pushing myself to do speech therapy to help improve my stutter. Something I can recommend that helped me when I was younger was listen to audiobooks as it showed me how the narreters carefully spoke in different tones and voices to be better understood by listeners. If you really want to step it up a notch, practice speech therepy techniques by reading a book aloud to your child then letting him read aloud some of the book as well. Also from someone who didn't have the best support growing up with a stutter I have to say you are a wonderful parent for being patient and supportive with your child. I hope all works out for you!