Why would this NPC give up his claim to the throne?

[NewJeffCT]

Here's another suggestion: if the PCs are to help fight off the evil warlord, they're going to need help of their own. Perhaps they're travelling with an ambassador from the old kingdom, who would have his own honour guard - they get involved with the ambassador earlier in his trip when he hires them to do something discreetly for him. Or maybe they stay at the same village / inn when it gets attacked by gnolls / bandits - the ambassador's force doesn't need the PCs' help to defeat the force but he's impressed by their heroism / nobility. Whichever way you swing it, the PCs turn up to the siege with a not-so-small force of highly competent warriors, and it's the combination of the PCs and these warriors that breaks the siege (maybe the ambassador & his warriors make a feint while the PCs go for the leader). Then both sides get to make a good impression and you can take it from there.

Lastly, listen to the players: if they make a good suggestion, let it work.

I was going to start with something along those lines - have the PCs handle some of the bad guys' heavy hitters during the siege (i.e., the blackguard himself, some of the badder gnolls, and the Goristro, of course...) and having that make the paladin favorably indisposed to the PCs.

However, the paladin will still feel that independence for his nation and his people is the best option for them. Perhaps if the son journeys with the PCs on the next leg of the journey, he can see that his father's best option might be unification with the old empire... provided the son maybe marries into the royal family?