I remember in primary school I was quite bad at English, spelling, grammar etc. My teachers advice to me was to practice writing, so I wrote a book. I started writing it from grade 6 through to grade 12, it ended up being over 900+ pages long! It is still very interesting for me to read the fantasy story because I can see how my english improved along the way.
The best way to improve writing is to practice it. Just like piano, the best way to improve is not to read about it but to actually do it. Write short stories to start with, perhaps get him to write about his own life and family, or his favorite things. You should encourage him to be detailed in what he writes, that requires that you read his writing and then question what he wrote for him to encourage him to question his own writing and thus make improvements. It is the ability to question what is written that makes a good english student, of course as well as the basic grammar, spelling and structure of sentences/paragraphs etc.
I used poems to help with interpretation and reading "secret" meanings of text. Because poems are almost like the music of language it encourages us to find a personal connection with what we are reading, it encourages us to envisage things very fast. Poetry can be read at face value but often that is very dry and boring, if we can see deeper into the text we appreciate it more.
There is no right or wrong when it comes to reading into texts. There is only you trying to prove yourself, to lay out the evidence. With my english students I use to always say, "I don't believe you, you haven't convinced me that what you say here is true, you cannot expect me just to trust you." This encourages them to elaborate and try to persuade the reader of their work with evidence and a logic.
But as a primary student I would really aim to get them to be able to spell good, construct sentences and be able to read well. Writing essays, constructive critique of works and interpretations of texts usally is developed in highschool. The no 1 advice I can give is to get him to practice writing every day, even if what is written sounds bad don't worry. Simply write without trying to improve what you do. Your first draft should always be simply writing and not thinking about how to improve what you write. Then get him to go back and try to improve what he writes, that might require you to prompt questions with what is written. Don't give answers in English just ask questions constantly, that helps a lot more than simply telling them. Sometimes your questioning must go extremely close to an answer if the student cannot work it out, but you find eventually you questioning can become more vague and they do things more on their own.
Get to know the texts that interest your son, then you can try to encourage him to question why particular things interest him. Why does he find things funny, perhaps ask him has he ever seen that happen in real life before. Ask him what he would feel if it happened to himself or someone else. You can perhaps ask him to write something about the funniest things he has ever seen and why it effected him so much.
You should try to get him to critique anything he sees on television (movies, ads, news etc) or the newspaper. Not to take things at face value, perhaps he can realize that sometimes media aims to promote a certain value and attitude that may not necessarily be the "truth".