He's in the 50th anniversary special in November as a "forgotten/hidden" incarnation between Paul McGann and Christopher Ecclestone who did bad stuff and got disowned - at least that's what it looks like so far.
The shots I had seen showed him in the same costume as McGann - that typically means it is the same Doctor, or a new Doctor just after his regeneration. So, it may be that he's playing the same iteration as McGann, who has just been around long enough to age. Unless you have some script-level intel?