@Mary Terry, it sounds like he communicates very well!
My cousin is developmentally disabled. She had brain damage at birth and at 56 is thought to have the cognative abilities of an 8 year old. But she is also largely non-verbal. But unlike your nephew she lacks sophistication in her communications. Still, I watch her get frustrated over her inability to express her thoughts and I wonder if there isn't far more going on in her head than the doctors give her credit for.
I wonder if every non-verbal child or adult had their own Anne Sullivan to teach them how to communicate if they wouldn't thrive.
I believe that most nonverbal people are "in there" and have a heck of lot more going on in their heads than anyone gives them credit for. They just lack the tools to express it. Perhaps some simply don't want to communicate but so many are frustrated by their inability to do it.
My nephew has evolved a lot over his 30+ years, and he is getting better at expressing himself. I'll always help him any way I can. I love him.