OP sounds like a perfectionism issue to me. If he's obsessing over making perfectly-formed letters, that can cause his writing process to be derailed.
My daughter was dealing with a similar issue recently. It wasn't her letter-forming that was the issue, it was her spelling, which isn't up to the level of her reading and verbal skills. So if she had an idea in her head she wanted to put down on paper, she knew by reading it back that it was wrong, but she didn't know how to make it right, and that turned her off from the whole process.
I'm not sure what flipped the switch with her, but suddenly she loves writing.
