Book Series Help For Young Adults?

The 11 year old in the family likes to read, a boy, and wants to read a series but he can't find any that look interesting. He reads at a high level, so no kids books. He is looking for good series' but a good single book he would appreciate. Any ideas?
Update: Thanks! Any more anyone?
Update 2: Also, he doesn't like the knights and armor and stuff too much. Realistic fiction more? I don't know. Whatever you guys have. :)
Update 3: Rainbow's gravity...? Huh?
Update 4: He's in 6th grade but reads at 8th or 9th grade levels.
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