I am not a professional but what you are describing sounds exactly like a gifted child on the spectrum to me - huge vocabulary but no pragmatic language or interest in communicating with others. Avid interests - but entirely self centered/driven, with zero interest or tolerance for other peoples ideas, interests or activities.

I understand that you feel he's happy in his own world and only unhappy when in therapy - but to me that's kind of the point. While he definitely needs time to just be, and to be happy, he does also need to be learning the skills to function later...

His therapist doesn't sound that skilled so a full evaluation sounds like a good plan.