My daughter is/was like this at the same age, including the “faking” because as she tells it she has fun saying things differently. It comes across quite aggressive when you say you ”accuse” your son of faking. I hope you mean you question and explore his motives. I told my daughter that I sometimes have trouble saying words, so I wanted to know if she was taking after me, which is ok. That’s when she explained she was having fun. Her disfluency has lessened significantly since that conversation. I think he will outgrow it, but be mindful if it worsens.