I just asked my "panel of experts," my 4th grade and 7th grade children. My 4th grader sounds a lot like your son. He reads well above his grade level, but is not interested in reading. Stories have to grab his attention immediately and hold it, or he won't bother. Here are their recommendations. All of these are fairly quick reads:
The first book of the Sisters Grimm series, The Fairytale Detectives, is relatively short, a quick read, and lots of fun. Plus if he does get interested there's an entire series he can follow up with.
Would his teacher accept a report on a graphic novel? (I would, but some teachers will not. You'll need to ask. I find well-written, well-drawn graphic novels to be a great gateway for reluctant readers.) If so, try the Amulet series. The first book is called The Stone Keeper. These are richly illustrated, well-written, and interesting.
Has he read How To Train Your Dragon? The reading level may be a little simple, but the story is original, engaging, and quite funny. And it is very short, which could be helpful if your son is still dragging his feet about doing this as you get closer to his deadline.
Lloyd Alexander's The Book of Three is brilliant. It is short, well-written, entertaining, and thought-provoking. It is based on the folklore of Wales, and along with a great story provides a chance to reflect on what actually makes someone a hero. It is a stand-alone story, but is part of a five book series, The Prydain Chronicles, so if you son does end up liking it, there's more.
Time Cat, also by Lloyd Alexander, is a more light-hearted time travel fantasy. A boy discovers that his cat can travel through time and goes with him for adventures. You see, cats do have 9 lives, but what that means in the story is that they can travel between lives in 9 different time periods.
My daughter recommends Coraline and The Graveyard Book by Neil Gaiman. My son has not read them yet, so I don't know how well they would fly with a reluctant reader.
Another thought - how would you (and his teacher) feel about reinforcing/supplementing his reading by allowing him to listen to parts of the book on CD? This is a tool I used to help my son get happy about reading, and for us, it was very helpful.
Good luck!