I have just joined Mumsnet just now in the hope that I could find some advice for my dyspraxic teenage son. I was relieved and also upset to hear about your son. I can completely relate to what you are going through.
My son is 14, and has started to go into full teenage meltdown. He has hardly been at school since Christmas as he gets up most mornings so emotional that he is physically sick, and now refuses to go to school. I have to go and see his school tomorrow to try and get something sorted, as I am at my wits end.
The defiance doesn't stop at going to school though, as you say, it's getting him to do anything or show any interest in anything is a complete nightmare. A is in 3rd year at school, with exams next year. I am thinking that it will be a longer process for him, and he may not get all his qualifications at school, and he might have to do some at college. A's progress does seem to take longer than his piers, but he does get there, and it may just be the same for your son.
Good luck with the book, I'm away to look into it myself.
By the way I'm told A will get a scribe for his exams, you should look into this too if you haven't already done so.
Sorry I think I've rambled a bit here.