Is this likely to help or hinder my case? I don't want to be seen to be making excuses...
With the supporting evidence, I don't think it will hinder your case.
i also don't want them thinking that the pressures of a Grammar school could trigger further problems. Although the GP stated that the right counselling now would help him prepare himself mentally for future exams eg GCSE's etc.
It would be helpful if the GP puts that in the letter.
If I were to play devil's advocate, the panel might wonder how he's able to do so well ("by the end of year 6 will be 5a's-6's
") with so much anxiety/depression/absence.
You, of course, will want to argue "Look how well he's done. He'll do even better when he gets all the help and support he needs!
You could also try pointing out that the 11+ was significantly more traumatic for him than routine school work.
Is this a state school? I was under the impression "levels" have been replaced.