Jason was a resident of Adytum and was in a relationship with the leaders of the Blades, Razor. Jason was murdered by the Regulators on the outskirts of Adytum sometime prior to the Vault Dweller's arrival. Their motivation to do so was to have a reason to rid the Boneyard of the other "gangs" outside of Adytum, and the Blades in particular. Caleb, the leader of the Regulators, also wanted to have Jason killed because of his relationship with Razor. One night, Jason tried to sneak into town to see his father, but the Regulators brutally murdered and decapitated him, before placing his head on a stake for Jon to see, with the intention of framing the Blades. The plan worked perfectly, as Jon believed the Blades responsible for his son's murder. However, Razor is in possession of a holodisk that shows the conversation between two Regulators prior to Jason's death, and the truth about what happened the night of his murder. Jason's death serves as the cataclysm for the conflict between Adytum and the Blades.
Jason's death serves as fuel for the conflict within the Boneyard. Believing the Blades are responsible for his son's death, Jon pleads the Vault Dweller to kill Razor, their leader. Depending on the actions of the Vault Dweller, the Blades can either be destroyed or the truth can be revealed to Jon. When Jon discovers the true perpetrators, he is quickly murdered with a shotgun by his personal Regulator bodyguard, triggering a battle between the Vault Dweller and the Regulators.
Jason Zimmerman does not appear physically in any Fallout game. He is mentioned only in Fallout. He was supposed to appear alive in Fallout, but he was cut from the final version of the game. His dialogue file can still be found in master.dat, though it is unused.