Every stutter is so different that honestly the most realistic thing you can probably do is pick your own random triggers for him. Like for me if I can hear myself well my stutter is almost completely gone. I'm good at talking in front of groups with a microphone, but at a loud concert just talking to my best friend I'll be a wreck. Also when I'm cold it gets worse, or when I say something fine with no stutter but someone didn't hear and asks me to repeat myself I stutter a lot. So a lot of random situations make it better or worse. Pick your own triggers for the character, or feel free to use my triggers or any others you see mentioned on this subreddit