Are speech impediments caused by undertrained vocal muscles or is it all in the kids head?”
With 4 boys, and 2 of them having gone or going through speech therapy, what has been explained to me is that there are also the muscles in the mouth and tongue to consider. Because my oldest son didn’t like anything stronger than soft meats, the side-to-side movement of the mouth never quite developed and it was harder to understand his speech. My third son just spoke too quickly and didn’t slow down to actually form his tongue correctly within his mouth to form R’s (his oldest brother’s name is Ryan, so there was lots to practice.) I would really suggest having him visit a speech specialist for an initial evaluation then decide if it is something that you can do at home, or something that will need specialized attention. FYI, we did not have to endure therapy long. The earlier that it is addressed, the faster the child will learn, but either way it takes parent-consistent involvement. Good Luck!
