How do we know anything? I spend a lot of time around him, observed his behaviors and interactions with the world, and have taken away that he is more capable than some people understand. I’m not making a general observation, just a personal one, hence why I am asking for further studies to imperically show me the general data.

As a bit of a rant, what you’ve said is pretty silly in any context. Are you telling me body language and physical interaction are not inherent forms of communication? We evolved and worked together for millions of years without spoken word.