You can, but in so doing, the mummy character he would have would likely be stronger (of a higher ECL) than the others in the group. The savage progression VV is talking about basically spreads the mummy's abilities out and makes it into a playable class, gaining the actual racial abilities as it levels up. And it is about a 12 or 13 level class, thereby removing any Cleric levels he might get.
Unless you want to HR it, almight DM.