Why would anyone think that you have to "know" to reverse it? You don't! He says "That man's father is MY father's son. We already know he has no brothers or sisters (and how the O.P got that the picture WAS of brothers and sisters, I don't know!), so HIS father's son has to be himself. Get it? There's no need to actually reverse it, it still means the same thing. Like "Who is that man's father? I am, and I'm an only child!" Just a bizarre way of saying that. I've encountered dozens of these over the years, some pretty far-fetched sounding and near impossible to figure out due to the confusion, but this one IS pretty straightforward.
Thinking of it in "reverse" just makes it easier to figure out/understand, you don't actually HAVE to, and besides anyone who knows math well can think that out logically anyway, especially if they ever did any computer programming!
I found this forum when searching around for codes and ciphers, but I have always been mathematically inclined. In high school I did my best to take as much math as they'd let me, and always made A+ on everything. Algebra & algebra II, geometry, trig and I wanted to try a little calculus but couldn't as I had to make up TWO history classes to graduate! I kept failing history as I felt it a waste of time when I could be learning more math and science, which I could ace easily.
Even as a young kid, I was fascinated by things like square roots, and eventually started learning more about codes, ciphers and encryption which became almost an obsession. One day, I was fooling around with some things and I got an idea for a "code" of sorts that would be simple for me to read without any tables or machines, but if you knew nothing about it you didn't stand much chance of figuring it out.
Now, some 40 years later I STILL use it to make private notes and such without having to hide them, and I had even thought of running a contest just to see how much work it would be to crack it. My plan was to charge a fee to join, and whoever got it first would win back their entry and most of everybody else's. I'm sure I'd need to use some of it for promotion and publication or something. I'd release a phrase or sentence and allow a certain amount of time. If nobody got it, I'd release a little more (like most, it's easier to crack the more of it you have!), and so on until someone got it. It's pretty good, but I'm sure somebody would crack it soon. Funny thing is, the whole code came to me when thinking about other ways to represent English and I was actually looking for ways to improve on what we have (which, you must admit, is a mess!).
Well, that's me in a nutshell. I'll be popping in from time to time!