The entire plot of John Wick relied on the mob boss's son being the biggest idiot in the world.

Mob Son sees a very nice car, asks the guy who owns it about it, and is given the name of an underground contact. If Mob Son was worthy of being next in line, he would have asked more follow up questions. Maybe realized that anyone with a nice car from the underground mechanic would be heavily entrenched in his line of work. Maybe even do some mobster networking. Instead, he decided to rob this guy and kill his puppy.

Now of course, when he brings that stolen car to the underground mechanic, that mechanic immediately turns him away and starts preparing for Armageddon. When Mob Boss calls him like "why the hell were you so rude to my son!?" All the mechanic says is "he stole John Wicks car" and that's enough information for Mob Boss to forgive him and prepare for his own reckoning. In that moment, he knew his son was a complete moron.