@bobbun_ He can live, but his basic physiology is still human, when he becomes a head other principles enter into play but until there his body needs all that a human body needs, and even if he can continuously recover from the lack of oxygen it doesn't mean he isnt affected by it.
@bobbun_ because if I remember correctly, they said his regeneration is linked with his thoughts. So if he can't think then he can't regenerate. That would then mean he needs his brain and the brain itself needs oxygen to be able to operate properly.