...so that it appears to him he was gone much longer than the amount of time that passed on his ship.
I think you mean that on Earth, more time would have transpired than it would have for the spacecraft and its crew.
Going off memory here, but a spaceship and crew leaving Earth to travel to the center? of our galaxy and then back to Earth would take it 20 years. On Earth, though, 2 million years would have passed.