It would never be Pocket because no matter how they spin the wheel on that one, he's a Kirakis. (if his parents are Philip/Mimi or Shawn-D/Belle) Since Sonny and Leo had sex, it rules out Leo secretly being any biological member of the Kiriakis or Johnson family.
And please note, when I mentioned Zach (with an "h") in a prior post, I talking about Zach Parker, a child Megan Hathaway used to trick Bo into thinking she had given birth to his baby when they were in high school. He was not Megan or Bo's son. And he is a completely different child than Zack (with a "k"), the son of Bo and Hope, born nearly two decades later.