He sounds like me. I got affected by the change around me. New school, new class, new teacher, new kids, etc. It’s more of a confident issue. Try to let him play sports as much as possible, then workout when he reaches that age. The more confident he feels, the less stuttering he will get (hopefully). Also, please don’t focus on his stuttering, but always pay attention to him. He needs listeners who don’t make a big deal about his stuttering.