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.
I will look up examples online for my own benefit. Thanks for the suggestion. In his K class last year, his handwriting was consistently about the worst in the class vs his peers. The teacher would place examples of the other children's writing up on the wall and DS's was almost always illegible. Or, it would be a marker outline he would trace. He also says his writing isn't as good as the other kids. He still doesn't color in the lines or draw beyond a simple stick figure for all his pictures.
