In Marvel’s G.I. Joe comics, Cobra Commander’s son is Billy.
Billy Kessler first appears as a boy caught up in Cobra’s operations. His father abandons his family and takes Billy with him, exposing the child to Cobra’s dangerous world. Billy later becomes the target of a plot by the Baroness and Major Bludd, who train him to assassinate Cobra Commander during a public rally.
Destro discovers the plan and prevents the assassination, forcing Cobra Commander to acknowledge Billy as his son. Storm Shadow then helps Billy escape and trains him in ninjitsu. Billy ultimately rejects Cobra and becomes connected to the Arashikage ninja clan and, in later continuities, the G.I. Joe cause.
Billy’s story is unusually tragic for a franchise character. A Cobra attack leaves him missing an eye and a leg, but he survives and continues fighting. “Serpentor” and “Rex” are associated with other Cobra history, not Cobra Commander’s son; Billy is the specific answer in the Marvel continuity.