Following this with great interest. My DS15 has always had this and it manifests just in conversation (which can appear as, and, in fact, I think has resulted in some social anxiety) and in his writing. If you take the time to listen to him and wait for him to retrieve his words or you ask for more and more elaboration and detail (because he will often resort to saying what he wants to say in the least number of words possible to shorten the process) he can eventually show you that he has vast knowledge of which he speaks or writes. Unfortunately, this has had a huge impact on school. He has never been diagnosed, though.