My son took a CTY writing course over the summer and enjoyed it very much.
I've met the teacher of
www.onlineg3.com and she seems wonderful - obviously has really thought about how to engage advanced readers.
It seems like a local tutor might be great for writing (look for one who works with adults and they may have fewer preconcieved notions about what kids of X age are supposed to be doing) - other tips are getting the child emailing to relatives, blogging on forums of interest, or writing articles for wikipedia.
For reading you may be able to attend a Library book club or start your own. A friend of mine has attended a 'Parent and Child' bookclub for years and that seems really cool.
Good luck!
Grinity
