I swear, the /f/ is going to be the end of me. Although according to the Phoneme Acquisiton Data from Sander, I shouldn't be worried. Still, as with all things with Christian, I'm constantly in defense mode.
Here's a couple of things that we've noticed with the /f/ sound in general:
-It IS an emerging sound as he DOES produce in ALL positions. Just not consistently.
-He has trouble producing it as an initial sound if it preceeds a vowel. It often is substituted for a /b/ or /p/ in these cases.
-/f/ in the final position in single syllables is a strength, so we're building on it!
We're way past the whole "scraping frosting/nutella/marshmallow fluff/ of the bottom lip". We've moved onto some great exercises passed onto us by both Christian's school SLP and our new itinerant teacher. One fun exercise that helps build the /f/ in the final position that Christian LOVES uses plastic little spiders from Halloween that I place on his arm. As he says "Off", we flick the bugs off his arm. This really has helped him with correct placement of his top teeth on the lip and his pressure.
We've been working on this for a year. It reminds me of potty training---tedious, but knowing the end result is going be worth the effort.