If your son is an average reader, have you thought of the idea of Dysgraphia? A lot of what you describe sounds like Dysgraphia. Writing down letters backwards, or in the wrong place, horrific spelling, can't copy but can read a paragraph. All are signs. Does he also have really bad handwriting? Doesn't use punctuation well? More signs.
My best of luck to you.
p.s I developed Dysgraphia at an older age because of disorganized thought processing by another disorder. I can read fine, a little slow reading out loud, but can pronounce any written word, and can comprehend what I read (to an extent of an ADHD mind 
).
SGH