I'm a 64 year-old mom who has worked in S.F. counties daycare systems and my own daycare for infants and then still found the energy to have my boy at 45. Just so you know where I'm coming from (lotsa wonderful babies, of all kinds and parents to match) Parent child communications is a specialty of mine.
I just want to say that you are doing the right thing. You are a loving concerned mom. Please don't worry; boys do internalize their emotional responses often and store up ideas and pictures in their minds before they use the language part of their brains and marvelous minds. Each and every child is different; your son speaks more often,at his age, than some very intelligent and wonderful young ones I've seen grow from birth to manhood.
I was particularly pleased to imagine him saying all those "sh' words! Isn't that great? It shows how he is categorizing by means of tactile information with his sense of touch in his mouth. Sometimes little guys make up words and as you suggested we don't know what they mean! I knew a little boy whose parents and friends, including an older sister spoke many languages. He had a hard time with language and was much more into physical exploration pouring water and stacking things just like a little student of life. For a while when he was two and a half he gave up trying to say all those different words people were using and made up a word for milk. "Flortch." he'd say pointing to it when his mom was puzzled. It was really a very smart move. he drew attention to the fact that he felt everybody else was just making up words as they pleased, so why shouldn't he?!
Please don't pay much attention to those books. Comparisons always make us feel defeated in one way or another. If we match what they say is ok, we feel dependent on their approval. It we don't match we feel inferior. Just read those books with a special hat. I call it the expert hat. I always stop and think, "Now wait! I'm the expert on me and my child." before I take in more than I should of anyone else's opinion. No matter how scientific and perfect the study none of us can second guess the individual .
The way I could tell you are a loving mom is how you said you know he understands you and does what you ask him to. This shows he loves and trusts you and wants to please you. One of the ways you can give yourselves the gift of a long happy relationship is to show him that you're content with who he is and think he's perfect the way he is. Holding your child and showing him things silently, enjoying looking at and touching things like sand and toys and natural items, flowers,feathers, even if he takes them apart, will create a silent friendship between you the way it does when two old friends sit quietly enjoying nature together.
Remember boys excel at language systems and reading only when they have reached 10 or 12 years of age. He will be pushed too soon to start using language in ways that are difficult for all little kids. Don't add feelings of worry to that task. For now he is safe at home with you being signaled in all ways that he is wonderful. Clearly he is storing verbal information. You don't have to worry about that. There may soon come a day when you have to ask him to stop talking for a moment; you'll probably get a laugh out of that time.
Much love to you and yours Andrea