I might be giving away very personal information, but I have stuttered my entire life. Sometimes extremely severely, sometimes you won't even notice. I have been to speech therapists.
So, some tips. First, do not react to your child's stutter. Your child is probably already aware of his stutter and self conscious. Do not tell him to take a breath, do not tell him to slow down. Do not finish his sentences unless he gives you permission to. In most cases it is a bad idea.
Do not focus attention on his stutter, in other words.
Contact a speech therapist immediately and get him assessed. If he begins therapy soon, he might outgrow it and not stutter as an adult. I did not outgrow it.
Different people deal with stutters in different ways. Myself, it has never really stopped me from interacting. I talk to random strangers, I give presentations, and I make phone calls, even though I really don't like the phone. I have had people laugh at me, had people tease me, had people ask me if I forgot my name. Usually it is just ignorance - people unaware of what a stutter is. I always participated in class, and put my hand up in class to answer questions if I knew the answer.
I don't why I did, or why my stutter does not really affect me socially. I think my stutter is perhaps not as bad as some - sometimes I am pretty fluent, even on the phone. Some people suffer from terrible social anxiety relating to their stutter, and this can really affect them. Be careful to make sure that your son does not develop such anxiety.