Oh, I was that kid once! The only real memory I have of elementary school is my whole class yelling at me because they got mad at my stutter, lol. Everyone else has provided really excellent advice, especially to befriend the speech therapist & to see if you could potentially give speeches to your son's class about stuttering. Or recommend other resources to your kid's school to teach their class about stutters and other speech impediments. They're only 5 years old—ableism is taught and can be corrected with proper education. I'd also recommend looking into a regular therapist for your kiddo. Ideally, someone that has experience working with disabled kids, or specifically kids with speech impediments. It'd really help to prepare him and teach him how to work through these situations in the future. Ableism can be really traumatic and isolating to deal with as a kid.