I always wondered when it was decided to make wilard decker and matt decker(the doomsday machine) were father and son,it wasn't mentioned any where,so can someone explain it for me.
It originates from some behind the scenes info and is included in the TMP novelization written by Roddenberry.
My understanding is that it was an idea that was kicked around in the writer's room, but maybe they might have been hesitant to mention it directly for fear of having to pay some kind of royalty to Norman Spinrad to name drop his character? Even the Phase II series writer's document makes no mention of it. It does say both his father and his grandfather were academy graduates, but no specific mention is made of Matt Decker by name. Roddenberry's novelization obviously decides to just roll with it and make the connection, but nothing concrete seems to have been nailed down during the actual making of Phase II. So while nearly all of us accept it without question, technically the direct connection between both Deckers is "soft canon" at best.
The Deckers — A new sitcom on NBC this fall! Is there a fleet big enough for the two of them? Young Decker thinks dad's old fashioned and to obsessed with his work. Dad suspects his son is a lefty pinko. What will happen when dad finds out his son is involved in interspecies dating? Hijinx ensue!
Star Trek II: The Wrath of Momma Decker "James Kirk! You bastard! You killed both my husband and my only son!"
Even though there is nothing explicit in canon, you can be sure fans will have a collective fit should some future Trek call them uncle and nephew, or not related at all. Remember the fanon (eventually disproved by ENT) that the Klingon Bird of Prey was actually a Romulan design, because an early STIII script had Kruge steal it from the Romulans?
The Phase II writer's bible that was published in the Reeves-Stevens's book makes specific mention that his father and grandfather both being Starfleet officers "of flag rank" before him, but makes no specific mention of Matt Decker by name. Was this added in a later revision?
No, I think there was only one edition. So the connection must have been made in places such as Susan Sackett's "Starlog" column, "The Making of TMP" and Roddenberry's novelization.
The Reeves-Stevens's book does publish a memo from (I think) Jon Povill where he mentions Will Decker being Matt Decker's son, in response to the Harold Livingston first draft instead claiming his father was the captain of a merchant vessel (IIRC). The crux of Povill's memo was a friendly note along the lines of, "Hey, I thought we'd talked about his father being Matt Decker from The Doomsday Machine?" or something to that regard. But it definitely never made it into the bible. So the idea was clearly being kicked around the writer's room at least, even if it didn't make it into the writer's bible itself. But as you said, Gene Roddenberry did end up making explicit mention of Matt Decker as being Will's dad in the TMP novelization, so on those terms at least the link between them is direct and specific. I know it was one of things that got talked about a lot at the time though, so even if it wasn't in the writer's bible then it was evidently a part of the character background they were discussing at the time.
At the time of the events in "The Doomsday Machine" Will Decker would have already been an adult and serving in Starfleet somewhere. He might even have had his first command---given TMP takes place supposedly little more than two-and-a-half years after the Enterprise's 5-year mission. So Matt Decker wasn't just a contemporary of Kirk's, but rather he was likely quite a bit older than Kirk.
^ I know it's always dicey going by actor ages, but... It looks like William Windom is seven and a half years older than William Shatner, and he would have been 43 when The Doomsday Machine was filmed. (Shatner was 36, and was playing a 34(?) year old character.) But you're probably correct, and Decker was probably older than Windom, and just looked younger due to increased 23rd century lifespans. (Much like Picard is much older than Patrick Stewart.)