My ds used to stutter and has only stopped in the last couple of months. (He's 8 yrs.) He'd been doing it for about 18 months and has had severe speech delay.
I really panicked because it's not unusual in pre schoolers but it is less common the older they get.
It used to be so bad that sometimes it made me cry watching him struggle. The school were aware and were fantastic for not putting him on the spot. He also saw a speech therapist.
A lot of people say ignore it but as Lavenderhoney has said, it's not very helpful. We made a point of never interrupting him or saying anything negative but what we did do when it was really, really bad was to get him to stop what he was saying, have a little think and to try again. (Bit easier to explain as he's older.) But my ds wasn't worried by his stutter so that approach worked for us. But that would depend on your ds. If it was his 'normal stutter' we were just patient and waited for him to finish with no interruption from us.
I am also a fast talker and have had to slow right down when having a conversation with him, still do make a point now.
Our school were brilliant and the amazing thing was that no child in his class ever picked up on it. So he never lost confidence.
Anyway last week he was signed off from the speech therapist! [hsmile] We did work out that his stutter was worse when he had learnt new words and was trying to use them in conversation. Once he worked it out, the stutter would ease off. Like what Greensleeves has mentioned - their mouth struggles to keep up with their brain.
Tell his teacher your concerns, so that she doesn't expect too much from him and maybe get referred for SALT.
Good luck! x