I think that, contrary to what you said, it would actually be quite typical for the main character’s stutter to lessen in severity as the story progresses. For many people, a stutter worsens when they are stressed or emotionally affected in some manner. As the main character’s confidence grows and he acclimates to being a leader, the stutter will lessen - just not permanently. It’s not a static thing for the majority of people and improvements are not always linear either. For me personally, my parents obsessively spent time making me improve at public speaking, debating, rhetoric etc and I had a constant preoccupation with the way that I spoke because everybody else also did. Most people get used to it though and, as a result, its actually easier to speak with them. I think its more about perspective (internally, esp if you’re writing from first person, and externally) more than some objective list of behaviours or characteristics. I’m not sure if this will be useful for you. As an aside, based on the setting of your story and the plot, you could consider researching historical figures in positions of leadership/rulership who had stutters and compare how they are presented in different sources and what they achieved with how you wanted to characterise MC.