My experience with my son was similar. It was the day that I actually saw him get FRUSTRATED with the fact that he couldn't complete the word that I was alarmed. After getting stuck on "wha wha wha wha wha...." he said, "Uh! I can't SAY that." I asked him, "Are you having trouble thinking of the word you need to say or are you just not able to SAY it?" "I can't SAY it," he told me. So, I said, "Do you have a question about this book?" "Yes." "Do you have a question about this frog in the book?" "What is the frog doing?" he asked me. I asked if that was what he wanted to ask me and he said it was. He was CERTAINLY able to ask me those sorts of questions before.
Anyway, I took him to a local public school and had him tested for speech. He went to speech for 2 years and now is in kindergarten and goes to speech once a week. He seems to go through periods where he doesn't stutter at all, and then all of a sudden, he will stutter very noticeably.
These are the coping techniques that help with him: If I notice him stuttering, I will STOP what I'm doing, and look at him in the eye so that he knows he has my full attention. Then I will try to slow down my speech a little bit (not like a robot, just a little more deliberate in my words.) If I am looking at him and he is STILL stuttering, I will say kindly, "Remember, I'm listening to you and there is no need to rush." It usually calms him down enough take care of the stutter.
My suggestion is to follow your instinct. I don't think that every child who stutters needs speech therapy. I only became concerned when it was noticeable and frustrating to HIM. I have found that some adults have been insensitive to him, and some older children, but his peers never even seemed to notice - which has also helped a lot.
Good luck!