Not his call. Every individual kernel developer owns their own contributions.

Even if Linus were to move his own contributions to GPLv3 and require GPLv3 for all new contributions, he'd still have to get permission from every single individual who ever contributed code from the kernel. Some of these people are dead, so he'd have to deal with their next of kin (or more likely, their lawyers), who might be even more difficult to deal with.