It’s normal for young children of that age to go through developmental periods of disfluency. However, those periods usually present as whole and part word repetitions, phrase repetitions and interjection words (ie ummm umm umm umm). Typically they do not present with prolongation and blocks (struggling to push the word out). I would push a little more with your doctor to get a speech therapy evaluation. They may not qualify him but it’s better to be safe than sorry. If you can’t, definitely don’t wait until 6. While its true that many stuttering kids out grow it by the time they’re 6, it totally depends on the nature of the stutter. I’m a speech therapist and always think it’s better to be safe than sorry and get a speech eval if possible! In the meantime, search “fluency shaping strategies” on YouTube. Easy onset, restarting the sentence, thinking before you speak are some of those strategies. Otherwise it sounds like you are already doing a lot of great stuff for him!