ditto pb. Your description sounds like he has trouble with idea generation, initiation, possibly organization and theme development. The WJIII does not assess any of these skills. These are actually more closely related to executive functions. Sometimes that may manifest as inconsistencies in written expression/output. A topic for which he has high interest and/or background knowledge may lower the threshold for the above EF-laden writing tasks.

...pronounced like the long vowel and first letter of the alphabet...