A Student Who Puzzles Me (or Where Is This Writing Coming From?)

David (not his real name, of course) is a student in my 10th grade class. What puzzles me about him is his writing, which varies from fluent to very weak (grammar, spelling, organization, development of ideas etc.). Whenever we do in class writing (journal entries, peer editing, project reflections, essays etc.) he produces very little compared to his peers and what he writes is muddled. He often asks to finish assignments at home. (Since he has an IEP, he is allowed extra time to complete work.) When he turns in these assignments, they are not perfect, but they are very well-written.

Here's what troubles me... I would love to believe that the extra time David spends on his work at home is the reason for the remarkable improvement in his writing. However, there is a part of me that suspects that he is getting help at home (but from whom?) and that the "stronger" writing is not (entirely) is own. He seems to do less writing in his other classes (Spanish, Math, Chemistry) so his teachers, all of whom I have spoken to, have less evidence of the difference in the quality of his work. We have not received the CAHSEE scores yet (he took the test untimed), and I am very interested to see if he passed the writing sample portion. I've mentioned my concerns to the Special Ed. coordinator and she understands my dilemma. How do discuss my suspicions with his parents and with him without seeming as if I am accusing him unjustly?