Returning to Grace
Though the character has turned his back on their deity, that doesn't necessarily mean the deity has turned their back on the character. Throughout the next few sessions, you could introduce signs that the deity is prepared for the character to return to the fold. Whether the character picks up on these opportunities or not is going to depend on the specific character arc, but one great motivating factor is treating the character as a lower level for that time (as they've lost their cleric levels).
In situations where the character completely abandons their former deity, I've used three different techniques.
The method that requires the least change is for the character to assume a new deity. This keeps them as a cleric, and at most changes the Divine Domain that determines their powers. The new deity may be a rival to the character's former deity, or just an opportunist. Remember, characters with the dedication to be clerics are rare, even among priesthoods:
Not every acolyte or officiant at a temple or shrine is a cleric. Some priests are called to a simple life of temple service, carrying out their gods’ will through prayer and sacrifice, not by magic and strength of arms. In some cities, priesthood amounts to a political office, viewed as a stepping stone to higher positions of authority and involving no communion with a god at all. True clerics are rare in most hierarchies.
...so a deity would be remiss to pass up the opportunity to establish a new hand.
If the character instead is completely detracted from the idea of serving a deity, a different kind of Otherworldly Patron can fill the same role. This would involve replacing the cleric levels with warlock levels. In this case, it could be the character seeking power to fill that hole, or the patron seeking out the former cleric.
Sometimes the relationship between warlock and patron is like that of a cleric and a deity, though the beings that serve as patrons for warlocks are not gods.
If neither of those options fit the character's direction, another choice I offered my players in the past was to dive into training with another class. Since your character is already a multiclass character, they could simply direct their attention into deeply studying the ways that their wizard training could fill the gaps that their god left in revoking the divine magic. While there are no official rules that monitor this sort of retraining, it is the simplest way to allow the characters to stay at the same level.