Convincing a Reluctant Mentor

Perhaps the MC goes on the quest without the mentor's help initially and the mentor's conscious forces him to intervene, as the mentor isn't totally devoid of emotion.

 

Well the plot point were the mentor decides to help has to be something that makes him realise it's personal.

Maybe the MC says something which reminds the mentor of how he used to be, and how the MC is like him, and how the mentor wished he had a mentor when he was like that.

So try to give the mentor a personal reason for helping: someone who he loves/once loved gets threatened, MC reminds him of himself, he gets inspired by the persistancy of the MC, he stumbles upon an ancient scroll that says the MC is 'the chosen one', some fairy makes him help the MC, it doesn't matter.. as long as it's not "Ok ok stop whining, I'll help you", it has to be believable.