As a literature teacher, I fully support the concept of reading 30 minutes a day. Not to insult math, but reading is THE MOST IMPORTANT skill kids learn in school, and the only way to really become proficient in reading is -- you know the answer -- to read. Practice is necessary.
Understanding that he doesn't prefer reading /language, I'd make these suggestions:
- Talk to him about the importance of a balanced education /being a well-rounded person. In 4th grade, he should be able to grasp that we're all allowed our preferences, but we have to become proficient in all the basics.
- Alternate books. He loves the silly stuff like Captain Underpants. The teacher wants something more serious. Alternate. Read a book of his choice, then it's time for one of the teacher's choices. This'll give him some of what he wants and will let him feel he's in control ... and some of what's probably better practice for him. As a literature teacher, I do think that novels are better "training"; novels require that you become familiar with a number of characters and follow a plotline across multiple chapters (multiple plotlines as you leave the Emerging Readers stage).
- He likes comic book type stuff ... ask him to create his own comics (hand drawn or computer-based) based upon the reading he's doing. He may not love the reading, but he may really enjoy translating it into comic form.
- Read what he's reading so you can discuss it. He may not be a fan of reading, but I bet he is a fan of one-on-one time talking to Mom and Dad. I know Harry Potter has been mentioned already, but NO BOOK in my children's entire childhoods provided us more discussion. The biggest topic: Was Snape good or bad? But so many other topics: What if Harry had been adopted by the Weasleys instead of his aunt and uncle? Which of the classes Harry took would be most useful to us Muggles? So many discussions, and I could see my girls' reading-ability grow as we debated. My girls may've had it better in that the movies weren't out yet, and we had to wait for books to be published ... so we had to read and rely upon facts from the books to support our ideas -- but other series are available too!
- Emphasize that by trying a variety of books, he may find another favorite that he didn't expect; for example, I remember that my two reading-reluctant brothers fell in love with Hitchhiker's Guide the Galaxy. Later both fell hard for Lord of the Rings.
- I loved the idea presented by a previous poster: Introduce him to biographies of famous mathematicians. I remember LOVING a certain series of biographies when I was in elementary school.
- Is he also a tech-y kid? Would he enjoy reading more if he were reading on a Kindle instead of paper books?
- If your family enjoys playing games, look into some word-based games. Scrabble comes to mind first, but my family loves Bananagrams. Don't tell him it's good for him.
Books to consider -- these are things my kids enjoyed in late elementary school:
Series of Unfortunate Events -- my oldest was INSANE about these dry-humored books
Bunnicula -- this has the same weird sense of humor as Captain Underpants
The Warriors -- a group of cats lives in a feudal-like setting in the forest, defending their turf from rival cats
Graphic novels of all sorts, though you might want to read some reviews -- these aren't all child appropriate
The Narnia series
Anything by Kate DiCamillo, though her stuff is a little girl-y
Running Out of Time -- about a girl who thinks she lives in American pioneer days; she doesn't. Was a huge favorite of my youngest.
Holes -- this novel has a nice circular plotline, which would appeal to math-science kids
Ender's Game -- Si-Fi novel, draws on video games and tech
Rick Riordan's Percy Jackson series
The City of Ember -- this is a distopian novel about humanity moving underground, and I could see the world-building concept appealing to a math-science kid
The Redwall series -- essentially this is King Arthur's story retold with forest animals; insanely popular among pre-teen boys
Vampirates -- yeah, vampires plus pirates -- what's not to love?
The Spook's Apprentice -- a little scary, but pre-teens love it