Hi mate, firstly kudos for putting a character like this in a novel. Ill respond to this as an avid reader, and life-long stutterer. As mentioned above by a number of people there are many different types of stuttering, and different triggers or sound obstacles. The typical sound repetitions which you've written so far, although immediately recognisable is fairly saturated in society and media/literature. Also, and this may just be my opinion but it can come across as forced and a little gratuitous at times. Also you'd do well to realise that once a stutterer has stuttered on a sound/word/syllable they very rare do so again in the same sentence. A more accurate way of depicting this would be Sutter on the Mr. But not Mrs. In julio's introduction. Practically, I doubt the attention span of your target audience would be enough to read through the M-M-M-M's and simply skip to the word. I'd suggest him being stuck on words and sounds rather than repetition. I'm going to suggest something which I believe will build on your friendship and personal growth themes, this also this corroborates with my own childhood. Have julio be a thinking character, quiet with everyone EXCEPT his friends. Have him be empowered by his friends with small amounts of stuttering and regular dialouge and small amounts of dialouge with people outside that circle. At the end of the novel have him begin to widen that circle and talk more aa well as grow in confidence as a result of this advebture, but the stuttering should not disappear. Just my 2 cents. Id be very interested in reading this when finished, also to offer any advice or further feedback.