A.,
My daugther did that from age 2-3. When I had taken her in for check-up I asked about it. they said it is very common. And everyone who has stated "their brians are thinking faster than their mouths" is very TRUE.
My doctor made a good point, if you don't make it apparent that he has this problem, and don't correct the words for him, and let him figure out what he is saying, it REALLY WILL GO AWAY IN TIME. Before you know, you will have forgotten that he even had a stutter. It will go away. Just make sure that you and your hubby kinda "ignore" it and don't ever finish his sentences for him. If you do, that makes him think there is something wrong, and then he will become aware of the problem, and might focus too much on it.
I know things will work out for you in the end. My daughter will be 6 this summer, and has never stuttered since. I did lots of research on the internet, and I read many places that it was totaly common and normal. I stopped worring and let her take her "verbal" course.
:)