I tend to be a little test-happy, because of my profession, BUT, I also have seen a very wide range of typical development in six-year-olds. I think the primary criteria I would consider would be
1. forward progress: is he making gains in reading & writing?
2. frustration: are his levels of decoding, handwriting, and written expression skills causing him to feel bad about his schooling, or interfering within his acquisition of, or demonstration of, other skills?
3. functional skills: are there any other nagging concerns that you have had about his development, outside of pure academics?
If you have ongoing concerns in one or more of these areas, then you might consider additional assessment.
In the meantime, you might try some very simple accommodations:
1. visual guide for reading: take a book mark, turn it horizontally, and have him use it as a placemarker under the line that he is reading in text passages. A lot of young readers find this helpful.
2. separate the steps of written expression: ask him to dictate sentences to you about the writing topic. Read aloud, or have him read aloud, his dictated work, to check for meaning, syntax, and organization. Make any corrections he voices. Then dictate the sentences back to him to copy into his assignment.
3. if the difficulty is idea generation, try asking him pre-writing questions about the topic, such as sensory details, or action words.
-Some children (esp. GT) have difficulty narrowing down the possible topics for writing. It's not really that they can't think of anything, it's that they don't know how to select an idea. In that case, offer some constraints--e.g., instead of "my summer vacation", "one activity I did with my family during summer vacation", or "the first fun thing I did during summer vacation". Try to make topics more concrete and contained.
-Other kids have difficulty with topics with absolutes ("the best", "my favorite"). In that case, change the absolute to a lesser level ("one good", "a favorite"), so he doesn't feel like he has to have comprehensive knowledge of the whole universe to answer the question.
