I’ll admit, it’s close but I do prefer climate catastrophe over The Zunker

You need a lot of company and knowledge and luck to survive without a bigger network (eg a whole country) and without task splitting communities (eg a whole country) supporting the little and big needs just for food, let alone clean water or medicine or hygiene or energy or defense or whatever.

That bunker is just a fancy way to die a little later. He may carry on for a year, maybe even two before lack of everything will kill him - if noone capable attacks that bunker which pretty sure will happen within weeks as it's a prime target after everything else had been looted.