Sudden Speech Substitution

by Lucy
(NY)

My 6 year old son starting speaking rather late, in fact, we had 2 evaluations done at 18 months and at 2 years because of this. His receptive language was above average, so even though he only had a handful of words, until almost 3, he never qualified for any early intervention services. When he did begin speaking, it was basically in complete sentences. So we were very relieved that it was all for naught. He had a few unusual sound substitutions, for example /s/ for /f/ like sish for fish and goosey for Goofy, but otherwise his articulation and pronoun usage and everything came right along, and most articulation errors corrected themselves.

Now, suddenly, he is substituting the /th/ sound for /f/. It's is so strange. It is mostly at the end of words - girrath for girrafe, stuth for stuff, and leath for leaf. However he has made this substitution at the beginning and middle of words- lathing for laughing theeling for feeling.

He recently finished kindergarten and all of his scores are at grade level, reading is even a little higher.

What would cause this sudden issue? Should I be worried? Should I be correcting him or having him practice? It just seems so weird for such a bright little boy to suddenly go backwards with his speech.

Thank you so much!