Turning into a bit of a boomer-bashing thread this now hasn't it? Not cool. Not ALL boomers are fucking rolling in it.
And I am not a boomer by the way. 100% Generation X! Yes of COURSE it's hard for young people these days, but the very idea that everyone born before 1965 has had an easy life, and everything has fallen onto a silver platter for them, is ignorant and presumptuous (along with also being bloody wrong!)
Maybe the OP's dad has been the butt of snide remarks for having the audacity to finish work. And THIS is why he makes these kind of comments/jokes!
A friend of mine retired early due to ill health - at 53 - after working 37 years. Almost immediately, she started to receive a barrage of snarky comments and verbal abuse, from bitter and jealous people (usually women around the same age as her within 3 to 5 years,) who still have to work. I mean, never mind the fact she has disability and is in pain half the time or anything!
So maybe the OP's dad keeps getting similar remarks and is on the defence.
@AnyFucker
Or those smug retirees I know who said “It almost makes me wish I still had to work again” on the news of an extra bank holiday for King Charles coronation.
I have literally NEVER heard anyone who is retired say 'I almost wish I was working again so I could get the extra bank holiday off.' Why would anyone even say that? It makes zero sense. 
Also @malificent7 as a pp said, why mention you do NHS shifts? 