In reading your initial post a few red flags jumped out at me. I keep thinking something else may be going on here especially since the school work issues began around December.
Couple of things:
Could he be depressed? 10 is the beginning of a difficult time for many kids. Would it make sense to have a professional assess him for this? Did something happen at school? Is he being bullied? Girl troubles? Is he doing this because he wants out of the gifted program (maybe doesn't like the kids or teacher)? Has there been a new teacher or student since December??? It just seems like there's something else going on here. In my work as a behavior analyst, when I run into such cases there's usually a new variable like a new student or something that caused the change. Just a thought.
I'm astonished that the school wont allow your son to type when hand writing is a known area of need. The one good thing I will say about our NJ district is there's almost no emphasis on handwriting. This has been good and bad, but mostly good in my DS7's case. I'm not familiar with the 74% rule. Maybe you could ask for a "due process" hearing and fight the school for accommodations. ??? In NJ if a parent of a child with an IEP drops the "due process" line, schools ultimately wave the white flag of surrender because they don't want to go to court.
My heart aches for you. Sometimes I worry that I will be in your situation if my DS7 doesn't get it through his head that he needs to check his work and not be careless. He doesn't have ADHD but he is the classic absent minded professor.
I wish you the best with this. Keep us posted. This is a wonderful forum, especially when times are tough and no one else seems to understand.