He isn't the Doctor at all.
He is the Master.
Yep, you heard that right.
The Master actually masquerades as a future Doctor.
It's a moot point by now, as he actually is revealed to be the Master for the credits of the recently released
Dark Eyes 2, where he has a wonderful line (from the trailer), mocking the Doctor about his battle against the Daleks...
What have you been DOING since you stole that empty TARDIS of yours, since you first landed on Skaro? [...] Shouting, "Look at me! I'm not fighting a war!" while you battle the Daleks all the way through space and time?