Originally Posted by CPRich

That's probably the last thing you should do. They may hold the plane waiting for him, continually ask you to try him on his cell phone, ask if you still want to fly without him, etc.

Just say that he won't be traveling today, plans have changed.

You should also inform the airline in advance. You can retain the credit for the ticket (maybe) instead of losing it if you don't cancel. And it also allows the airline to know one additional seat is available if standby pax are available.