He was notified of the reroute prior to the first leg but the actual reroute did not take place until after the airborne departure of his first leg. (He flew the first leg as scheduled.) So, technically, since he didn't fly the reroute until after the first leg, wasn't he rerouted after the airborne departure of his first leg.........?

23L2 doesn't state he can't be notified of a reroute prior to the airborne departure of the first leg, just that you cannot be rerouted prior...(with the exceptions)

Denny