Hi C.,
My son goes through phases of stuttering. It seems so frustrating to him but is often followed by an increased ability to form sentences and complex thoughts. Other parents often tell me they are amazed by his vocabulary (hooray for reading) - picking the right word and getting it out along with all the other words takes practice.
I wait for him to get his thought out and have found that trying to "help him along" by finishing his thoughts or sentence is only frustrating to him. Also, if anyone teases him about the stuttering I go right into protective parent mode and politely ask they stop teasing him ("oh, don't tease him! his brain is working faster than his mouth"). Sometimes people don't even realize they are doing it and they are just thinking it's kind of funny. But it's not.
Keep an eye on it and if it continues for months, seek the advice of a professional.
Good luck!
T.