He does not absolutely have to go to a land crossing - and he can do it at an airport. BUT, he has to get it while "entering" the US at a POE, whether land or air. That means that if he goes to an airport, he would have to fly out of the US and then on his return trip back into the US he can get his TN.
I know even at a land crossing, you have to physically leave the US and reenter to get a new TN.
I tried that once at Detroit a few years ago. I drove to Detroit and parked in the parking lot just before I went over the bridge and stepped over the guard rail and went into the US Customs office and they kicked me out and told me that I had to physically leave the country and re-enter to try to get my TN redone. I imagine that the same thing would be true at the airport. You actually have to be "inspected" upon admission into the US.