First, I think having the character having a stutter, but not having it as his defining characteristic is a great idea and will make for an interesting character. I think he should stutter less during less stressful situations and stutter more when he's excited, scared, or any situation where his brain would be paying less attention to his speech and more attention to something else. You may want to have him blocking on a few words. For an example "You've got to replace the ...(1.7 second pause)...Battery" I really like how his speech is worse when talking to the adults and better around his friend. Nailed it. I also thing he's stuttering just about the right amount in the provided example. I would put him on the 72nd percentile of stuttering severeness, but then again it tends to get better as you get older. All in all, very nice! Feel free to hit me up if you have any questions.