Elon was asked about a replacement CEO in late 2014, he stated that there currently was no one in mind, and that the plan was to stay on as CEO till volume production of Model 3 and the Gigafactory, and then evaluate [1]. Of course, he may not have been at liberty to say the person in mind.

[1] https://youtu.be/FE4iFYqi4QU?t=7m25s