Talk to your players (especially the priest). Let them know what you expect, see what THEY expect and reach some agreement. With the priest, ask him to create holy days, rituals, habits etc...they don't have to be detailed, just something he can reference. The idea is, with creation comes investment, with investment, interest. And he will want his own creation to make some impact and may actually start pushing other players (at a minimum, he should read deities and demigods for his own deity, paying attention to dogma and vows. You should too so you can know when he does something opposite.
Still with the priest, he needs to behave in accordance to faith. For example, dealing with cultist of some foul god should be much harder for him then dealing with evil assassin. Withdrawing healing (and support) from character who mocks his faith should be rewarded. Doing the same for mocking him (the character, not the player) should not.
I would never instruct my players how they should play their character.
He chose the priest. If he doesn't want to play one, he should choose something else. And my first advice was to talk to them
Other thread proposals: I wouldn't directly punish the player in a sense of withdrawing spells (except in extreme cases), but social effects, hierarchy and recognition slights (yes, you are heroes, you saved the city, but why would we, as a church, recognize someone who misuses power of the divine?)
Why would you, as a church, recognise someone who misuses power of the divine?
The polite reason is because we saved the city. And also that looks like a nice church you have there, it would be a real shame if something happened to it.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.