You might want to back off and reestablish a habit of best effort by requiring only one letter, but it must be correct and neat and look like the model. If he does it correctly, he’s done. If he does it incorrectly, he has to do it over the right way.
Once one letter is easy for him to do correctly and well, you can increase the expectation to two. Build up slowly, though; always giving him the responsibility/choice/motivation to do it right the first time.