I agree with Pigtoria's response. Both vocabulary and comprehension can be increased by your child listening to stories and then discussing it with you. If your son is having difficult identifying the words in the text, he would be able to comprehending it. Start with what he can do, which is listen. You can read to him or get recorded materials (check your public library, as it probably has a large selection of stories on CDs.) The nice thing about recorded stories is you can listen to them in the car.
The key is that you both hear the same story. Discuss the story elements as well as the vocabulary. Expose your son to different types of genres - mysteries, biographies, nonfiction, historical fiction, poems, humor, etc. Make it fun!
Once your son feels most comfortable with his reading, have him read sections of the book silently. You need to read the same section and then discuss with him both the vocabulary and elements.
I believe that everyone – children and adults – all have their weaknesses with reading comprehension. Standardized tests with reading passages present challenges for all test takers regardless whether they’re taking the CTBS, the SAT or the GRE. It’s a weakness that we must start working on at a young age by taking small steps.
Understanding the theme of the story is easier when it’s a short passage instead of a long story book and it’s easier when the story is being read to. When children read, it’s more difficult for them to grasp the main ideas of the book as they focus too much on pronunciation rather than the content.
To help your son, you could start with reading to your son short stories consisting of a couple of paragraphs. Ask questions about what was read after each paragraph or even after a couple of sentences if necessary. When you are confident he can grasp the main ideas of short stories, you can have him read the stories and see if he can understand it. When your son can understand short stories read to him and reading it by himself, move to longer story book. Scholastic books that are sold in package are great for short stories reading.