Hi A.,
My son had speech issues that were identified at 3 1/2. It was more pronouncing sounds and lisping. Your local elementary school can assist you in determining whether or not he needs intervention. They will set up an appointment and have him evaluated.
After my son was diagnosed, he was enrolled in a speech preschool that was a couple hours, twice a week. It was fun for him and he really enjoyed it. Once he reached Kinder, he went into speech at school. He stopped therapy at the end of second grade. Today he is in sixth grade and is a pro at speaking in front of the school for its leadership program.
At the time, I was very worried about teasing. I cannot tell you enough how positive his experience in the speech program was. The only teasing incident I can remember was at a friend's birthday party where an older child made fun of him. I witnessed it but so did the boy's dad. I am pretty sure that child has not made fun of many since. His dad was pretty ticked off. The teachers I spoke with said that most students are envious of those who go to speech. This seemed to be the case. :)
Perhaps your son's stuttering is part of his normal development. I know many little people's mouths can't catch up to the jumble of words swirling around in their brains. However, for your peace of mind, it might be worth it to check it out. In California all these services were free. Hopefully your state will have the same programs.
Good luck and try not to worry. I sometimes miss my little lisper and think back and smile.
C.