How do you deal with the thought of possibly never knowing your child truly?

My son is non verbal and 6 years old. He isn't aggressive or vocal whatsoever. He never cries, never laughs, never really shows and true emotion that I can decipher. I asked people when he got his diagnosis for advice on communication, and they said I would "just know" what he wants. That I would get an instinct for it eventually. I don't have that. I look in his eyes and he's in his own world. He is the sweetest boy but it breaks my heart that I don't even know his favourite colour or show. I play music for him and bring him to do activities and I can't tell if he loves them or hates them. He's just neutral. On everything.

He's a smart boy and professionals we have seen say he might speak one day as long as we keep making progress, but he hardly even makes a sound when I try to do linguistic exercises with him. I wish I knew what was going on in his mind.

It's frustrating. Not because he is hard to parent, but because he is so indistinct. I would love for him to just throw a tantrum or scream so I know he can tell me when he's upset. I can't stand the thought of him being sad and not being able to tell me.

I would love to know other examples of parents with similar stories and how you overcame the heartbreak of it all. I just want to make him laugh or do something to show him I love him further than what I have been doing previously.