If he's in boxes it's too late.
If there are still a few days before he'll be in boxes, it's not too late. Just talk to him tomorrow (unless he'll be gone by then) and tell him to stay.
If you haven't saved the game since you said goodbye, you can quit without saving, and let him ping you again and then you can tell him to stay. Or you can talk the next day until he brings up the subject of moving (it'll be something like "I'm moving in 4 days..I'm having second thoughts, should I stay?" etc etc), and then tell him to stay. It's not too late, unless his moving date happens to be tomorrow or something.