As a reading specialist, I never suggest workbooks. The best way to improve comprehension is to:
*read lots and lots of books and to talk about them.
*read and talk about books from lots of different genres (realistic fiction, non-fiction, fantasy, sci fi, biographies, poetry, sports stories, magazines, newspapers, graphic novels, how-to manuals).
*Read the same books with him. You don't have to read together, but if you also read the book you can have great discussions and model things for him. Maybe even get Dad or an older sibling or grandparent to read the book too.
*Think book discussion conversations and not book quizzes. Go beyond the who, what, when, why questions and ask though provoking questions. And follow up with how do you know or what evidence do you have type questions.
*Talk about vocabulary!! I challenge students to find words that they didn't know how to read, weren't sure what it meant, don't use with their friends, would put in a glossary, think is a cool word, or know mom and dad would say how in the world do you know that word! We collect words and try to use them in daily conversation.
*Talk about parts that were confusing for you (I often pretend a part was confusing if I think it was confusing for the student) and talk about how you figured it out.
*Ask questions like:
-What is the lesson/moral of this book?
-What lesson did the character learn?
-How would you describe this character? Go beyond physical characteristics and include character traits, motivations, and feelings.
-How would the story change if it was told from the perspective of a different character?
-Would you be friends with the character? Why/why not?
-Why do you think the character did that?
-How did this book make you feel?
-What part of the book surprised you?
-What was your favorite part and why?
-If you could change something about the book or rewrite the ending what would you change?
-Who would you recommend this book to and why?
A great resource I found is the web site Bookopolis. It's like Goodreads for kids. With a free account (that is very secure) kids can rate books, write book recommendations, and do "book reports". My own kids love it. You can read other kids' recommendations.
Authentic reading or real books with real discussion is so much better than a workbook. Workbooks are boring. The comprehension questions are usually lower level thinking questions that teach basic recall, but not deeper thinking which is needed to truly comprehend a book.