First and foremost, stop calling attention to your son's stuttering. Do not ask him to repeat a stuttered word. You could very well create the very problem you fear! I have four sons, and I did daycare in my home, too. I have lots of experience with this age range and it is completely normal. Some kids do, some kids don't. Two of mine did, two didn't. Many of the kids I babysat did this. They are all over 20 now and not a one has a stuttering problem. I am acquainted with a 21 year old young lady who does have a bona fide stuttering problem. It began, as most stuttering problems do, at the age of 4. Only time will tell whether your son has a real stuttering problem. You just cannot judge such a thing at his age because so many kids do it at this age. here is the important thing to remember. By calling attention to it and making it an issue, if he does wind up to have a stuttering problem, your actions could make it much worse. A child who is frustrated by a parent pointing out the problem may be more likely to tighten or tense speech muscles when stuttering occurs. Such tension may increase how long the stuttering lasts. Your response to his stuttering can aggravate his fluency difficulties. People who stutter vary widely in how they react to the problem. Some appear to be minimally concerned. Others—especially those who have encountered unfavorable reactions from parents and other listeners —may develop emotional responses to stuttering that hinder their speech even more. Shame, embarrassment, and anxiety will become associated with talking, and a minor stuttering problem will become a serious, lifelong disability.
It can be frustrating and time consuming to listen to a little kid who stutters now and then. I know! :0) But allow your son to get his words out in his own good time, do not draw attention to it, and chances are, he will outgrow it in a few years.