My DH stutters. He says that these are both sounds he gets stuck on, too.
If you research stuttering, there's a large percentage of kids who outgrow it naturally.
I don't know if what Charlie's doing is really stuttering, but sounds possible.
For my DH, the worst thing is when someone tries to finish the word for him. Be aware of wanting to do this...it's natural, it seems helpful. Be patient and supportive...not like you wouldn't be, but I'm just saying out loud what you probably already know.
~Kiyomi~ Unschooling mom to one very chatty, very amazing 7 year old bundle of boy