Oddly, I have just recently participated in a similar meeting on a student, although this meeting included written language and social-emotional assessment results.
I would suggest that, before you spring for the private psych and ed therapist, absolutely discuss appropriate assessments with the school. master of none has made some good suggestions. I would add social-emotional assessment, and would caution you that the visual-motor integration testing, at this age, with someone with good fine motor, is unlikely to find anything, because the developmental progression of fine-motor skills maxes out so early. I am also struck by the poem you mention, and some of your other descriptions, which sound more like what we would call performance deficits, rather than skill deficits, meaning the actual skill necessary to meet task demands is there, but other factors (attentional, behavioral, emotional, motivational, medical, attendance, etc.) interfere. A lot of different things can affect performance, but the one that leaps to mind in your account is social-emotional factors, such as anxiety and depression.
Being continuously bored can make one depressed. On the other hand, depressed individuals may identify their depressive sensations as boredom. Adolescents with depressive symptoms also often present as distracted/inattentive, angry, and oppositional. Unhappiness often makes people irritable and rigid. No one should diagnose over the internet ;), so I'm not saying this is what is going on with him, but I do think it would be as well to ask the questions, as much of what you have relayed would be consistent with this, and it would be good to rule it out, or be aware of any emotional fallout from a different factor contributing to the writing issues.
Another factor to look at would be the teaching style of his English instructors, and whether they allow him to express his verbal-conceptual strengths in classroom discussions; or shut him down because he talks too much, shows up other students (or the teacher!), or because his divergent thinking takes the teacher off his lesson plans. Or conversely, the classroom expectations are so rigid that a perfectionistic student feels anxiety about generating only flawless, fully-formed products, and is afraid to start, for fear of starting the "wrong way", or of writing something that will be ideologically disapproved of (hence the bland writing).
