I was thinking about your son today and I finally saw something. I saw that there are letters and words in your sons brain. But the pathways to get them to out of his mouth with speech are not strong enough. I'm seeing him with a woman therapist. I get the impression of a speech therapist. I see a lot of work using all different forms such as auditory work, tactile work. I'm seeing flash cards being used, a lot. I see a specific one of a tree. It has the picture on it and the word. I also see one with a frog. I don't know why these ones are so specific, but they are being shown to me clearly.
This woman is very nice. I feel she can help your son alot. I am getting the number 7. I believe it's an age. I think that is when he will really be able to start to communicate much more clearly. So I do think he needs more time.
If you have a therapist like the one I'm describing. (She's blonde btw), then you are on the right path. If not, then look for who I am describing. she is very kind. Her demeanor is super sweet and kind. She connects well with your son. It will be like she really understands him.
I hope this helps you. I was really struggling with your question b/c I really wanted desperately to give you something concrete to work with. I'm so glad it finally came to me.