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.
I like this and think I will try it. The groaning really escalates when I ask him to redo a letter. He will now self correct before bringing to me, so pointing out the good ones (and why) may be a better approach.
