Cleric leaving his Deity

Of course if you want to be harsh the answer can simply be "nothing". If he wants to leave the Cleric life behind him, he'll have to multiclass into a new class at next level up.

(This is one of the most flavourful answers, but can be one of the less fun ones.)

Back in 2e, Clerics were permitted a 'change of heart' exactly once with no adverse, long-term penalties.

If you'd like to add some flavor (and it's in the player's interest), require of him an Atonement spell from a high-ranking cleric of his new Deity. Maybe he has to donate to the church and/or perform a great service to earn it. Maybe such a service might include founding a new church/abbey/shrine/sacred space in or near a settlement.

The official D&D Adventurer's League allows a near-total 'reroll' of a character up to level 5. If your player doesn't want to be a CLERIC anymore, I'd suggest he be allowed to multi-class. If he wants to stay the same person AND be a new class, it's all in your hands, but better to let him play the character he wants.