In episode 9, "The Trip" we see Jack via flashback. We find out that he didn't know that William was alive and in his son's life. When a young Randall starts asking questions about his real parents, Jack felt it necessary to help him understand where he really came from.
|What Happened To The Father On This Is Us?|
Jack hires a private investigator to find Randall's birth parents.
It's amazing how Randall goes right into being a boy again around his dad Jack. Beautiful work @sterlingkb1. Beautiful son. #ThisIsUs. MV— Milo Ventimiglia (@MiloVentimiglia) November 30, 2016
Rebecca is adamant about her and Jack being Randall's parents. Jack and Rebecca both love Randall with all their hearts but they take very different approaches at solving the problem.
I understand Rebecca's reasoning but damn Randall has every right to be angry & hurt. How do you move on from that? #ThisIsUs— Fabiola (@Miss_Fabi_hola) November 30, 2016
Rebecca tracks William down to let him know that Randall was asking questions about him. William was five years sober, wondering how Randall was doing. Rebecca doesn't want Randall's parents to try to take Randall from her and Jack. Although Jack assures her that he would never let that happen, Rebecca shuts down all communication with William.