Depending on what the current water parameters are, I would be hesitant about doing a large water change right off the bat. If you haven't been doing regular changes, I imagine that the parameters are way off what your tap/well water is.
Although that is a big, tough fish, I would ease into the changes. Maybe start off with 30% every other day for a week or so, and then step it up.
Good luck with him, he is a beast.