I heard about this today on the radio, but I didn't realize his tumor was this bad. Terrible news.
And the tragic irony here is that his mother (who's around 105, if I recall correctly) is very likely to see her son die, which is always one of the worst things a mother can experience.
That said, if he dies or retires, particularly before the midterms, how would he be succeeded in the Senate? Would there be a special election or would the governor appoint someone to take his place before the next regular elections?
According to
http://www.ncsl.org/research/elections-and-campaigns/vacancies-in-the-united-states-senate.aspx, the Governor would appoint a successor of the same party as McCain, and the appointee serves until the next regularly-scheduled, statewide general election, which I assume would be 2018. The person elected at the same time would serve out the remainder of McCain's six-year term (he's up in 2022).