As an ex First Responder with a little experience, leaving him in his seat is not an option. There is ALWAYS a possibility that efficient CPR may have the right effect, and all large passenger aircraft have an automatic defibrillator on board.?? To leave him in his seat would be unforgiveable. No pulse or breathing does not necessarily mean dead, though if you leave him there, he soon would be!!.
Yes but .....
No pulse or breathing leaves a chance of revival if treated very quickly.
If you shine a bright light in the patient's eye and the iris does not contract there is only one diagnosis - the patient is either brain dead or totally blind in that eye. An airline captain would not be totally blind in both eyes. In more general terms, if both eyes fail to respond and there is no white stick or guide dog the patient must be dead.
If the pilot is dead and there is no replacement pilot to take his place then it is safer to leave him. If there is a chance of resusitation then it is worth moving him and giving all available appropriate treatment.