As things stand, he's being given nothing to lose by becoming de facto leader of the 'Leave' campaign, as Osborne seems to have the leadership stitched up in the event of a 'Stay' vote. That means that by joining the other side, he could build his support within the party to launch an opposing leadership bid.
Cameron is clearly desperate to have him inside the tent pissing out, though, so I wonder what he's being offered?