I have a little guy who will be in kindergarten next year who produces b/f. He produces /f/ in isolation, the medial and final positions of words PERFECTLY. However, when we attempt /f/ in the initial position of words he promptly substitutes a /b/. We have used the gestural cue of sliding our hands up our arms to "smooth it out," and he can produce an initial /f/ correctly when we pause (ex. ffff-eet). But without the pause it turns into a /b/.
I have EVEN tried to shape an initial /f/ from a medial /f/, which he can do! For example...
But he will say...
This is a forehead slapping moment. Any suggestion???