I can see why you feel that way. If you pursue additional testing through any avenue, you may wish to consider more in-depth writing assessment, such as the TOWL-4, which looks at different aspects of writing a bit more distinctly than the KTEA does. It is a handwritten test, so any impact handwriting has on his compositional skills should leave some kind of mark. As noted previously, this type of data should inform the development of his support plan, with regard to both accommodations and specialized instruction.
And one other perspective: a learner with sufficient intellectual resources and determination sometimes can pull together enough to generate a decent (or even high) quality product if it is not too long, and this effort does not have to be sustained over too long of a period of time. His classroom teachers have to contribute to the conversation as well. Their reports may be consistent or inconsistent with the 1:1 test findings. If you are particularly concerned with writing, request the presence of the English teacher or other person on his schedule with content expertise in written expression at his meeting. This teacher should help to put his skills-in-isolation in the larger context of expectations for quality output in his actual classes.
Even if he is found eligible, if the teacher (as is very possible) has not collected enough writing samples at this early stage in the school year to form a clear assessment of his writing, you can also consider an extended evaluation, to include further formal (TOWL-4) and informal (teacher observations) writing assessment. This would most likely (depending on your state) require the team to find him eligible, and either 1) generate a provisional IEP, which you would then sign, with the mutual understanding that it would be revised after the additional evaluative data was collected (usually in 4-8 weeks, again depending on the state); or 2) find that insufficient information was available to design the IEP, requiring the additional evaluations, in which case he would be on stay-put services until the new IEP was developed and signed.
