Dee,
From what I can imagine, you're little man is so excited to get the sentence out that all the words are filling his head so quickly, he can't process them quick enough into words & so he falls over the start of the sentence. Does that make any sense whatsoever? If you are worried, contact the health nurse, but it sounds to me like he is in such a hurry to get the sentence out that it's a bit wobbly in the beginning. My oldest - Jessica did this for a while, we used to just tell her 'slow down, have a think about what you're saying & then try again'.
HTH!
Jayne