I too have a 6 year old with a reading ability of greater than 10 years old (last time I was told, it was over 11 years old back in May, so who knows what it is now). I did not have him tested; his age has always been told to me by his teachers ever since reception because it is a relative outlier, thus noteworthy.
The problem is that although he can read at a level almost double his calendar age, his understanding of the world is very much at a 6-7 year old range. I fully understand JeffCapes desire to challenge yet interest DS1, but also am sympathetic to the child-led view expressed by ExcessAdrenaline.
In practical terms, this does mean that I often reach for "older" children's literature- not Enid Blyton and her ilk because a) I did not grow up reading these books therefore am not familiar with them and 2) they are culturally too remote to grab DS1's attention. The books I pick for him are ones I remember reading and ones my brothers read as 10 year olds (From the Mixed Up Files of Mrs. Basil E. Frankweiler, Harriet the Spy, the Superfudge series, The Indian in the Cupboard, Roald Dahl, Robert Lewis Stevenson). I also check the lists of Newberry and Caldicott Award winners (many of the books above were previous winners). Michael Morpurgo is also a well loved author of DS1.
However, DS1 is ecstatically happy choosing Dick King Smith (loves animal stories), the Wimpy Kid Series, and the Captain Underpants series. Not exactly challenging or edifying, but he really enjoys them, and frankly, I just care that he associates fun and relaxation with reading, not just "learning something".