I know others mentioned this in passing, but I wanted really to encourage you to go to your local library and see if they have a summer reading program for kids. Our library system has a summer program in which the kids read a certain number of books (they must keep a log of what they read) and then at the end they earn a coupon book that is terrific -- coupons for everything from professional sports game tickets to free ice cream to free passes to water parks and recreation centers, etc. Yes, it's open bribery, but it can work. Of course you'll have to be sure he's really reading what he says he's reading, and I'd add something to it like "For each book you read, you can only log it on your list after we talk about it (for maybe 15 minutes) and you tell me what happened, who the characters were, and what you did or didn't like about it," etc., so he's having to think about the book after he reads it, rather than possibly just zipping through it to put it onto his list. And as someone noted, bookstores often have their own reading programs too, with the rewards usually being more books.
Another option -- find him a kids' book club. Our library has these too. There may be one especially for boys; some areas do that.
And do find out what he really wants to read. Sometimes boys are much more interested in factual books about subjects they like (a sport, history, gross stuff in our bodies--yeah, it's science disguised as a gross-out book, etc.) Some studies have shown that many elementary aged boys really prefer factual books to stories that focus on emotions, conflict, family issues etc. I did checkout duties for my daughter's class of 4th grade 10-year-olds all school year long at our school library, and the boys checked out a lot of things like Guinness Book of World Records, various kinds of kids' almanacs and fact books, sports fact books, history nonfiction, as well as fantasy fiction like the Percy Jackson series, Charlie Bone series, Alex Rider series, etc. Anything fantasy or action was big with the boys this age. Even books that are really "graphic novels" are fine, according to our school librarian, if they get a reluctant reader to read.