Stuttering/stammering happens whenever you are in new environment or when you meet new people or in public speaking, probably in my case. Take your son out whenever you go for buying something and you can ask your son to buy the things by talking with vendor and stuff, so that he can gain some confidence in that way. Its is not a complete treatment, but it helps a little. For each stuttering person including me, some words cannot be pronounced and might be difficult to speak, you can ask your son to talk the word with you ( kind of practice) or find an alternate synonym,if it doesn't work. For permanent cure, even I am still looking for it but to my knowledge, gaining confidence ourselves will be one, I guess. Note: There are lot of books related to stammering. You can read some and get an idea as there will be some suggestions in it.