A.Q.
For my cousin, this was just a spurt in cognitive development. He went through speech therapy and at one point they wanted to do surgery to cut that piece of skin that holds underneath your tongue. What finally was decided was that his cognitive skills had outpaced his physical abilities. He was struggling to find the exact word while his mouth was trying to form a regular sentence. He outgrew it within a couple of years. But it was hard to watch him struggle. Just a side note, he has an IQ of over 160.
Keep up with the therapy and listen to your gut. It might be nothing you need to really worry about, but you know your son better than anyone. Trust your feelings.