Possibly ask him to look at his letters and tell you which ones he is most pleased with, which ones he'd like to redo... then support him and accept the level he sets? This may help him feel in control of his education and level of mastery and provide the benefit of ongoing sense of ownership of his decisions.
When I used to help kids learning to write, I'd always have them circle the letter they thought was best with a chosen crayon or pen. Then I'd circle the letter that I liked best & give very specific praise (eg, "This t touches the top & the bottom, & you put the cross line right in the middle."). Since you & your son are struggling so much, I might try this carrot approach.
Maybe you could back off on your expectations for rewriting letters until things improve... But I wouldn't ask him which letters he feels need to be rewritten unless you are willing to actually follow his lead. If you give him this power, and then override his decision by making him rewrite additional letters -you aren't empowering him; you're emphasizing his lack of control.
About the "10 minute" time frame. I'd let that go for yourself & him. Write down what you expect to get done in one session and check it off as you go. That way you can BOTH see progress. The emphasis shifts to what you have DONE rather than how long it took.
I'd also have a positively worded phrase to repeat during sessions. Something like, "When we're done with these tasks, we can go to the pool/library/park." (Positively worded as opposed to... "We can't do that until...") Phrases like this give me something to fall back on & help me concisely get my point across. It also keeps me from getting sucked into the arguing trap.
If all else fails & money isn't an issue, I'd go with the PT & possibly some more specific testing. Maybe you could do PT 1 time/week & the therapist could give daily "mini assignments" to complete on off days so you're not the bad guy?
Good luck!
