In fact, as an example, isn't there some often quoted statistic about half a percentage of all living humans having descended from Genghis Khan?
The idea about people coming from Genghis Khan is unrepresentative in a couple ways. First, that a large number of people have a common ancestor does not mean that the common ancestor was in fact Genghis Khan. It could have been someone else associated with the Mongol conquest.
Second, six hundred years from now you or I may have a lot of ancestors, assuming you have kids (I already do). There is nothing remarkable about that.
The closest I got to explaining it to him is that Advertising as a profession in the US is the successor to WWII psychological warfare. People are being paid to use your instincts and your worst impulses against you. All day. Every day. It started out weaponized, and now it's industrialized.
To opt out requires a level of enlightenment (literally and figuratively) that few invest in, and some doubt they could achieve even if it were a priority.
We have black hat hackers attacking our brains, and only occasionally do the white hat hackers get any sort of publicity, and some of those have less substance but are more approachable than the real experts. These people are almost always standing on the shoulders of giants, and thus their work feels derivative (eg, Marie Kondo), which is off-putting to people. Their style-over-substance makes them in effect hyper-palatable people, which is a form of cognitive dissonance with the message.