It may be with looking into in regards to a head injury. I'm saying this next thing to you as a father and a stutterer: be calm. Be calm for him. There are far more resources available to you than there were for my parents. How you deal with this can shape how he sees himself. My parents didn't deal with it too well. You may not want to think about this or hear it but you need to. Kids suck and he will be made fun of. Friends who don't care and accept him for who he is will be key. You will need to have a long talk with his father on how this needs to be handled. All of us on here can impart some wisdom from our experiences. I know I'll help if I can.