I typed a really long post, then realised I can make this question quite short and sweet.
Is there a body of experience out there of mothers who feared their Year Eight, perfectly bright boys would never really (a) grasp what's required of them in a decent academic answer, and (b) pull their finger out to provide it; and who were then proved wrong in time and whose kids came good and went on to get decent GCSEs, A levels... ?
Instinct suggests that this is a maturity issue, but could do with a bit of cheerleading here. Or, if your experience is that your bright boy just never really did well at an academic school because it just wasn't the right environment, maybe it would help to read about that too.
I feel his main problem is that he sits down to do it, but switches off mentally much too quickly and wants to finish and move on, so does not focus for long enough to produce work of sufficient quality (or indeed quantity, given that you need a bit of quantity to fit in the quality!!).
This will pass, right?