My man junior was revived from the radiation giving to him. At the end when godzilla dies junior wake up fully grown and takes the mantle.

Energy Absorption & Redirection: In the 1993 film Godzilla vs. Mechagodzilla II, Godzilla demonstrated another ability that may have been a variant of his nuclear pulse. After being struck with Mechagodzilla's Shock Anchor harpoons, Godzilla discharged a form of energy up the cables, severely damaging the robot's internal mechanisms. However, this was not the only time Godzilla ever used this ability. In Godzilla vs. SpaceGodzilla, Godzilla discharged energy through his physical blows in his offensive against the sinister space mutant SpaceGodzilla. In Godzilla vs. Mechagodzilla 2, Godzilla also demonstrated the ability to give off huge amounts of radiation and heat after absorbing Fire Rodan's energy. This heat proved to be so intense that it caused Super MechaGodzilla's synthetic diamond armor to literally melt, allowing the mech to be easily destroyed by Godzilla's spiral ray. Godzilla also has a capacity to emit various forms of energy. Godzilla is able to weaponize his own nuclear power in the form of his atomic breath or nuclear pulse, and constantly gives off strong nuclear radiation that is lethal to humans. In Godzilla vs. SpaceGodzilla, Godzilla emits energy through his punches to overload SpaceGodzilla with power and cause him to explode. In Godzilla vs. Destoroyah, Godzilla attempts to revive Godzilla Junior by breathing energy into him. This does manage to briefly stir Junior to life after he is seemingly killed by Destoroyah, but only temporarily. When Godzilla melts down, he imparts his energy into Junior's lifeless body, reviving him and mutating him into a fully-grown Godzilla. ​@ivanriverajr5012