What if he retired from the seat and they had a concurrent special election?
IIRC, that's never done anywhere. Anytime there's a vacancy in a Senate seat, the governor always appoints the replacement to fill out the term. It may be in the Constitution that way.
http://en.wikipedia.org/wiki/Seventeenth_Amendment_to_the_United_States_Constitution
When vacancies happen in the representation of any State in the Senate, the executive authority of such State shall issue writs of election to fill such vacancies: Provided, That the legislature of any State may empower the executive thereof to make temporary appointments until the people fill the vacancies by election as the legislature may direct.