Brendan Buckley's Universe and Everything in It by Sundee T. Frazier
Brendan, according to his gram is "chocolate milk"---chocolate from his dad and milk from his mom. He's a good kid with a good stable family. One day when he's walking around the mall with his gram he sees this old man at a table full of rocks. He strikes up a conversation but when his gram happens up on them she goes bezerk. Turns out the old man is his grandfather, his mom's father, who he's never met. His other grandpa recently passed away and so Bren is determined to learn more about his grandpa and to find out why they've never met.
Strong writing. Nice amount of allusions to things like the internet and video games---makes the setting and story more real, but not over the top. Just part of the story.
The only thing I didn't love was the ending. A little bit too After School Special-ish for my tastes, but those feel-good-endings were my tastes as an 11 year old, so there ya go.