Hi Steph,
Have you looked at Hoagies (http://www.hoagiesgifted.org/)? It is a great place to start your journey in gifted education. If you have never been there I recommend starting with following link.
http://www.hoagiesgifted.org/gifted_101.htm

Like your son when my dd was five she was able to read at a higher level than the books she preferred to read. I came to the conclusion that part of the reason was visual. The lower level books have more interesting pictures in them and even very gifted five year olds like pictures. I found some wonderful books at a higher reading level with beautiful pictures in them which were a nice bridge into normal chapter books. A great series of visually stimulating chapter books is the Geronimo Stilton series. The author uses fun, colorful fonts and lots of pictures. The series is popular with boys and girls.

Regarding writing, that issue gets complicated because it involves a motor skill. Many gifted children do not like to write because of the physical aspect. Their hands cannot keep up with their brains. My dd's composition improved dramatically once she learned to type.

Summer