system
Given the following post, respond as if you're commenting on Reddit:
human
[Doctor Who] How did Captain Jack Harkness turn into the big head?

Also, how did he manage to live for so long without the Doctor realising who he was?
gpt
I assume the power invested into him by Bad Wolf faded over time, and his immortality similarly faded after dying so many times.    
   
With partial regeneration and immortality, scars and wounds would start to persist, and the most vital part of the body, the head would be the least damaged, and the most protected.  
   
Eventually, the scars and equipment keeping him alive would have heavily disfigured him.  
    
As for the Doctor not noticing, the Face of Boe has existed for a *long* time. 
Long enough so that everyone had forgot the name Jack.    

   
And the Doctor knew Boe for a lot longer than he knew Jack.  
Boe was likely just saving the knowledge that he was Jack for a rainy day.  
It's easy to keep a secret from somebody when you know their future.