Out of all the Foxman children, Paul shares the most in common with their late father, Mort. He's stoic, macho, and—to be honest—not always pleasant to be around. Of course, he also shares his father's good qualities, like his hardworking nature, dedication to family, and loyalty. With Mort's recent death, Paul is being asked to fill his father's shoes more than ever before.
Metaphorically, that's like sticking your size-eight into Shaq's shoes and trying to walk around: a lot easier said than done.
Mort's personality lands somewhere in between John Wayne and Clint Eastwood. Minus the revolvers, of course. Mort stays "in the background" behind Hillary's wild theatrics, his cool demeanor providing a foil for his wife's absolute madness (1.19).
Paul doesn't merely share his father's personality—he also followed him into the family business. You might argue that Paul didn't exactly have a choice in this matter, but the fact remains. Most important, Paul is passionate about continuing his father's legacy. After all, he's "given the last ten years of [his] life to this company" and has no plans to slow down (18.51).
But Paul can't just imitate his father. Instead, he has to learn how to get the rest of the family involved too. At first, he simply wants to buy everyone's shares of the business, thinking (rightly, perhaps) that he should be the one in charge. By the end, however, he realizes that getting Phillip and Judd involved will only strengthen the business, as well as the sometimes-shaky bonds between the three brothers.
You have to remember that Mort was a pretty open-minded guy underneath it all. Would a stiff-lipped square really get married to a wild hippie like Hillary? Would a macho dude really be okay with his wife starting a relationship with her female best friend? Paul needs to loosen up—and he needs to realize that his father did, too.