Thank you for trying to do something over the summer to help his schooling!
I work with a lot of students (in middle school) that struggle with reading, and I admire your determination to help him.
You didn't mention it, but does he get any assistance during the school year? Has be been evaluated by the school for learning disabilities or dyslexia? You might want to look into this when school starts once again.
I agree with the others that you have to check it BEFORE he gets any rewards... and if it is obvious that he is just putting any word down, he has to do it again.... and again, if necessary.
Is there someone else that can do it with him? Maybe hire a "tutor" (HS student wanting to go into education, maybe) that would be willing to work with him? He may work better with someone else..... I know you've already put money into the program, but if he is going to fight you for hours, maybe having someone else do it would work better.
First thing in the morning would be best.... set a routine..... get up, eat breakfast, relax for 30 minutes, and then start on the work.
No discussion, arguing.... if he sucks you into the argument, he 'wins' by taking it off his shoulders.. (hard to explain what I mean, sorry). Don't yell, just restate 'You need to do this worksheet BEFORE you get to do anything else. End of discussion, period." Just state that calmly and rationally, and just keep repeating it if necessary. If you feel he will start destroying things, remove everything in his room, if necessary except the mattress on the floor and a desk. Make him earn the things back, if necessary.
I like the idea of reading a chapter book together..... I frequently read with students... they read a paragraph or two, and then I read a paragraph or two. Find a book that interests him.
Again, thank you for your determination to help him!