It depends a lot on their reading levels. What springs to mind are:
Diary of A Wimpy Kid is a cute series. Geronome Stilton is another group of books that are interesting to read. Chronicles of Narnia books are always good. The Warriors series is fun to read (a series of books about a society of cats). Charlie Bone series.
He's towards the top of his class and BF reads to and with him every night he is at his house (1/2 time-joint custody).