L. -- Good for you for working with your son! Great replies so far, and yes, you do need to encourage while not becoming "teacher" in his eyes. A few suggestions to add to the ones already:
--Many friends with boys tell me that elementary-aged boys tend to like non-fiction, fact-based or humorous books much better than fictional stories with interpersonal conflicts, emotional issues, etc. (in other words, Guinness Book of World Records and baseball player biographies beat out stories for many boys at this age). Though we adults would perhaps rather they read high-quality writing and good fiction, it's fine for them to read what interests them -- they need to see that books contain stuff they are interested in, even if it's stuff we as parents aren't crazy about. This means your son (depending on his personal interests) might want to pick out sports fact books, or Star Wars chapter books, or Pokemon fiction books. Not fun reading for the adults but if it keeps him going to the end, it's still reading--better choices will follow. Cartoon books like Garfield collections are also big with this age group. Of course you want to mix in some challenging and classic books for him, but remember, get him hooked on the idea that content is fun.
--Try reading some kids' mysteries to him if you haven't yet. The Boxcar Children series is still great (and some are updated to sound more "today" with the kids using computers etc.), and there are series like A to Z Mysteries and Bailey School Kids mysteries. Leave him with a cliffhanger when you stop and ask him what he thinks will happen next, then when you start next time, remind him about his prediction and see if it happens. This also helps prepare him for first and second grades when increasingly he will be asked to predict outcomes in stories. Plus he may just love mysteries.
--Library systems and bookstores have summer reading programs where the child fills out a form listing books read and gets a prize after reading a certain number of books. If he thinks of that as too much work, set up your own summer reading program, maybe requiring fewer books than these do and offering a prize he really loves (favorite restaurant dinner, mini-golf game, toy, etc.) when he has read X number of books and can tell you which he liked best and why, which he liked least and why, one favorite character, etc. A fun chart could help with this.
--Spend some library time where you just hang out and you sit and read your book quietly while he sits and reads his. Reading at the library instead of at home means fewer distractions, no toys or friends nearby to tempt him away, etc. And you can just read next to him without reading to him or having him read to you.
--Try magazines, available at libraries. National Geographic Kids has great facts, science and nature articles, etc., though it does carry ads; Ranger Rick has tons of nature and animal articles with terrific photos and is ad-free; Sports Illustrated for Kids gets high marks from my friends' sports-loving boys; Highlights is the all-time classic kids' magazine with articles, poems, comics and short fiction but no preaching (and no advertisements!). He might fall in love with magazines as my daughter has.
--Sit down and read the newspaper with him, especially trading off reading comics to each other and, if you take the Washington Post, read him the Kids Post page that appears daily (if you feel the content is OK for him -- it does sometimes deal with news-based issues that you might feel he's not ready for).
--I love the idea one person posted about having him help you cook by reading recipes, etc. Also, have him write, be in charge of, and read your grocery lists -- he reads them to you as you go around the store. Write out simple directions to a place you've never been and have him read them to you from his seat as you drive and you can legitimately say, "I can't read and drive at the same time, I need your help!"
Above all else, as you read the comics or hang out at the library etc., don't say "It's time to practice reading" but approach it with him as "This comic is so funny--check it out!" or "You love soccer--did you see this book about DC United? Look at this picture..." etc. Sorry to go on so long but this is a vital topic to me. Good luck and let us know what happens!