Call the tower and fire station at his final destination and give them a heads up. Tower can have all the local controllers congratulate him. Fire can roll the trucks for a water salute. Can also call Gander (or kzak, etc.) and have them send a send a cpdlc congrats message or selcal him a special congrats. Call his chief pilot and make sure they are aware it's his final flight and someone will be at the gate to congratulate him. For my old man's 37 years of service, I made sure everyone that I could contact made his last flight special.