Damn, can you imagine that being your first day? I assume he had already been through some kind of training or had some assistance in making the decision to ground air traffic
Terrible first day, but he had a senior position at New York TRACON (which controls nearly all air traffic in New York) for years before getting the federal position. He was definitely experienced.
If it did, it could only have been a positive effect. From a logistics standpoint, what happened that day is nothing short of astounding. Less than 3 hours after making the decision, the ENTIRE US airspace was cleared of all commercial traffic. 4500 planes were re-routed and grounded at a time of extreme uncertainty. It may have been his first day, but the guy did his job flawlessly.