Sounds like task-avoidant perfectionism to me. We know it well in our household.
Two things have helped with our DD in this area in the past:
1) Setting a frame of reference for reasonable expectations. Your DS is getting help from you in writing, so he knows what your writing looks like. Does he know what typical first-grade writing looks like? If you can't take him through the school and show him samples on the wall (I assume not, because summer), see if you can find some images online.
2) Setting up for a win, and then celebrating the win. Set a goal he'd have to work at but you KNOW he's capable of (you most certainly don't want him to fail, as that just makes things worse), push him though it, and then congratulate him. And then, don't forget to say, "I told you you could do it!" In just about any other context "I told you so" is considered rude, but for your child, it helps them to recognize that you know things they don't, and they should trust you in the future.
For the short term, it sounds like you've got a good approach, but the situation is not improving. There's no silver bullet here, because every child and every situation is different. Sometimes all that's needed is perseverance, because these kids tend to struggle until they have an "AHA!" moment, and then their obstacle disappears. Sometimes it does help to get someone new involved, because your DS might relax and not worry so much about pleasing the other person, or they may have a new way of presenting things that makes more sense to him. So your outsourcing instinct is not a bad one.
Another idea is that the task avoidance may be enhanced by the tediousness of the task. I mean, seriously, who wants to write four lines of n's? Anything you can do to enhance the environment might pay off... background music, some snack crackers handy, whatever.
For the long term, I suggest you pick up a couple of books on perfectionism for each of you. I could recommend one for the child, but it's specifically geared to girls. Since he's still working on reading, I'd read his to him, and discuss.
