No. The more times they get to see him, the better evaluation they'll have of him. And the more comfortable they become with him, the more confident they will be to pursue him. They will not be as interested in someone they have seen only once.
Your son will not be perfect or perform at the top of his game for every event. Coaches know that. But if he can demonstrate a certain consistency, that will be to his huge advantage. They will look at his capabilities and it sounds like he's off to a good start on that issue to have received a follow-up letter.