E.,
Talk to a doctor and then talk with a councelor. Both private (tutoring) and at his school.
I HATED reading when I was younger and still find myself struggling with books. I'm 26. I lose thought. It takes to long. I can't clear my mind long enough to focus on the words. I get confused on the characters. ect. Some may find this silly or think less of me but you know, it's true. I had to work 3 times as hard in school as most others because of reading. Especially in college.
Perhaps he is feeling some of the same stuff without saying it. Maybe he can't pinpoint why he doesn't like it. I liked my mom reading to me, but not being left with a book. It was boring and unstimulating. If he is unstimulated, this will cause negative feels to flurish.
Maybe don't punish him so much and set times for things so he understands his place. He comes home from school, has a snack, does his homework. Goes and plays outside. Comes in. Reads a few chapters of his book until dinner. Gets to watch tv after dinner. (just a for instance) _ i know easier said than done.
Also, something that bothers me:
My husband had ADD and his parents were told he probably wouldn't graduate high school. So, they never expected anything from him and basically treated him as a disappointment (not that you would ever do that)and tried to force him to do things he didn't want to.
He did end up graduating by a night program, but the fact of the matter was he stunk in english and excelled in science and math, therein was the problem. He is now a firefighter paramedic! He does complex math equations in his head on a regular basis to administer drugs and to use the fire truck.
He is a wonderful and brilliant. He knows everything about computers (he fixes plenty of them) builds websites, can cut a hole out of a solid wall and install french doors with no manual nor having done it before, ect.
So, you step son will have to fight himself, and the negativity that may surround him with the ADD issue.
Just know, the more you force it, the less he's likely to do, unless you are there reading it with him.
Best of luck,
J.
p.s. does he like writing? I love writing and still hate reading. Odd eh?