I'd find a few pieces of writing from school that look the most independent and free form - the furthest from fill in the blank possible. Show them to the tutor, and ask for an assessment of the difference in his performance of independent vs supported. That is, with support, how much better does he perform? What kind of support is needed? What relative weaknesses come through in the independent work? If the tutor as a F&P level on him, get the letter and a record of the assessment.
With stealth and stealth-like dyslexia, we've found that assessment is very difficult once ceilings on CORE are hit. Has he done the TOWL for writing? It's a good test with fairly finely separated skills.
