Magnum Mania!

On episode one of season four, Magnum is lost at sea. He keeps on flashing back to his youth and memories of his father. At the end of the episode, we learn the Thomas' dad died in Korea in 1951 and was buried on the fourth of July. On a later episode that same season, Thomas reminisces about a baseball game he attended with his dad in 1956. That should have been rather difficult since his dad had been dead for five years.

surely he talked about his stepfather Frank Peterson, his mother`s second husband. (see Season 6 Ep. 7 "Going home")

I don't think Magnum would ever call Frank, Dad! There were mistakes and changes in the show! His mothers maiden name was Sullivan, that is where his middle name came from! Only later did they change that, to make him a jr, than a III. Remember his real Dad played grandpa Sullivan!

Yeah, he would never call Frank "Dad", so it would have to be a flub. Frank was just "Frank" (or "The Jerk") to Magnum. The show had several flubs involving dates (including Magnum's own birthday).