I always imagine Bucky calling Steve “Stevie” and Steve absolutely hates it because it makes him feel smaller than he already is (but he never tells Bucky that). So at first Bucky says it all the time to mess with him, though after a while he notices just how much it bothers Steve so he stops almost completely.
After the serum though, Steve doesn’t seem to mind as much. Actually, it makes Steve feel secure. It’s a reminder of his old body, a reminder that he’s really just that tiny kid from Brooklyn. It’s a comfort, just in case he loses it. In case his new power does go to his head, to have that reminder. So Bucky says it a lot more frequently (even though he doesn’t know why Steve doesn’t mind anymore)
Afrer Bucky starts regaining some of his memories, he never says it anymore. The last time Bucky called Steve “Stevie” was the day of the train mission, their last mission together before the ice. Steve knows there’s something missing, he isn’t sure what. But even if he knew, he wouldn’t push it. Partly not to show how much he likes it, partly not to mess with Bucky’s already fragile memory.
It doesn’t matter how long he waits, Bucky never says it. Not in the first week back, not in the first month, not even in the first year.
Bucky has been fighting alongside Steve for a while. And the Avengers, but mainly Steve. Whenever Steve turns around, Bucky’s there behind him, watching his back. Even so, not every fight ends perfectly. Steve wakes up in the hospital after a particularly bad fight. He sees Bucky’s familiar face and the first thing he hears after he opens his eyes is Bucky “Stevie, you had me worried there for a while”
It’s the first time since the train that Steve has felt anywhere near whole.