My son sounds very similar and is also considered non-verbal. He was diagnosed at 4 and will be 12 soon. It was a struggle for me at times, cause I would get really frustrated when he was melting down and I didn't know why. Then when we went in for an assessment, a developmental pediatrician explained he was cognitively at the level of his age. I started thinking about it and realized how hard it must have been for him. To understand everything and not be able to express what he wants. I realized some of his meltdowns were because he was trying to say one thing, and another word was coming out. Or that no words at all were coming out. At that point I switched my focus from trying to get him to talk to trying to listen better. We find alternate ways to figure out what he's asking for. It's a lot of " can you show me?" or him saying " come with me" he speaks in movie quotes sometimes. Sometimes it means something. Sometimes he just likes saying it. It's ok if the words are wrong. It's ok if all he wants is someone to repeat it. I always want him to know I am listening.
We pulled him from mainstream school and sent him to special needs private schools at age 7. In hindsight, we assumed incorrectly that they would make a greater effort to educate him. Now at 11, my wife homeschools, because we can work on the things he needs and meet him at his level.
My wife and I call what you're talking about "grieving the child we expected". At some point we examined or expectations for what his life was going to be and had to let that go. I am not saying we gave up on him. I am saying we accepted he may live with us for the foreseeable future. I do not say this to scare you because your son is only 6, but his language may change but he may still be considered non verbal into adulthood. At some point you need to process that, otherwise it will eat you up inside.
He will be ok. He will live a fulfilling life. All we can do is prepare him the best we can, and love him for who he is.
Things that we have found that helped were researching, apraxia, echolalia, and gestalt language. They are all things a speech therapist should be able to help with. ABA went a long way for my son and making friends, but he also generally isn't interested in other people.
http://www.vimgenius.com is great for learning the basic commands. https://www.vim-hero.com is great if you want to go more in depth and play games to practice using them.