bobby returns (they had to cremate his remains, the lab tells athena, you understand, can't risk the virus spreading!) except combo virus+fever, his brain had a bit of a hard time, and has amnesia. No memory of who he is, or maybe before he joined the 118 (for added misery: maybe before his family dies? whose gonna tell him?), but regardless, bobby is back and recuperating somewhere. Maybe buck puts him up until they can figure out how to break it to athena, et.

anyway, bobby, either fresh from the grief of killing his family/finding out his family his dead/not being allowed to see his family (because they are dead), could be forgiven for mistaking the soft-eyed young man who seems to eager to help him anyway he can... bobby could be forgiven for mistaking buck for, well...look at him. Tall and filled out and eager to please. Bobby's made worse mistakes when drunk, and has let himself make a couple of times when the mood or self-loathing strikes.

basically: bobby mistaking buck as trade, and daddy issues buck, whether he would want it or not, can't bring himself to let bobby down again.