If he is still enjoying fiction aimed at younger readers, how about Tom Sawyer? You can even read it online if you don't have it - just click here. This is a great read for all ages, and if he enjoys it and feels ready for something more adult the logical next step is the more complex and serious, but still entertaining Huckleberry Finn.
Another great choice for a 12-year-old would be Daz 4 Zoe by Robert Swindells - a modern classic and very popular as a school text. For older classics you could try Of Mice and Men or Lord of the Flies - these are both intended for older readers though, and are very dark in many ways, so it very much depends on the boy's character. Another good read, though the language can be challenging in places, is Oliver Twist, and you could also try a selection of short stories - the Just William series may be a little young (though I know plently of adults who enjoy them) or some Saki stories go down very well with boys that age - try The Lumber Room, The open Window or Tobermory.