Can't find a way to link a PC's backstory to the main plot.

Not that the backstory is bad. No. It's fine. The character in question is a Paladin and while the backstory defines really well what the character is, it falls short in terms of plot hooks. I don't mind having a hero go through a psychological journey, in fact I like this better. But the player in question doesn't seem to want to do that. So I'm left with this well defined character that doesn't have any link to the main plot and the only use I've made of his backstory for now was some unrelated bad guys blackmailing him to collaborate or else his father will die. (Long story. If you want me to elaborate, tell me in the comments). If you're wandering, the main plot is about defeating a world-wide Orcus cult that are trying to retrieve all the pieces of the...blah...blah...blah...in order to...blah..blah...blah.

This really bothers me since all the other PCs have a link to the main plot and have had encounters build around that:

• The Ranger was dumped by his ex-partners who joined the cult and almost left him for dead. He now wants to find them all and get his revenge and/or understands why they did that (the player as well as the character is divided on that).
• The Rogue's family was killed by an Eremite (powerful Kyton) that the Cult is trying to get support from. He now wants to stop them from doing that as this is an extremely dangerous creature and it getting plane-shifted to the Material plane would be a disaster.

The Paladin is a very faithfull and caring guy that wants to make the world a better place and is ready to die in-order to do so. Simple and to the point. He fights the Cult because they're evil. Why wouldn't he fight them?

As I've said before, the character in itself is fine. It's the lack of plot hooks that annoys me and, I'm afraid, will annoy him. I've got entire plot lines based around the other 2 PCs' backstories and I can't help but think the Paladin will get annoyed that he doesn't get any.

Am I worrying too much?