Make him care

[AzraelX]

Don't shove decisions you think he should make down his throat. That's the opposite of what role playing is.

Pretty much what I was going to say.

Everything in the OP suggests that Joe isn't being allowed to roleplay, and any attempt he makes at roleplaying is deemed "not roleplaying" because of the DM's desire to roleplay through Joe. If anyone is roleplaying the Paladin, it's the DM, and Joe isn't allowed to deviate from that: thus Joe isn't being allowed to roleplay. "Doing whatever I personally think he should do" is not roleplaying, and "following a predetermined script" is not roleplaying. These things are the opposite of what roleplaying is.