It sounds like you are doing a great job understanding the needs of the
boy in your class. Have you tried sentence frames? I don’t know if he
can read at all ( but you can also do it orally). You would give him a basic
sentence structure and have him repeat/read it, but fill in the blanks. You
said he does not say complete sentences. So you could give him the
frame “I______at the park”, or “I ______ at the ________.” and he would fill
in the blanks, for example.
For writing paragraphs, I do the same things for my struggling students.
I write the paragraph, and have them fill in the words to make their story.
As we go on in the year, I provide less and less words, and usually after
much practice, they are writing 4-5 sentence paragraphs themselves,
because they seem to get the idea of the structure.