A veteran QB "grooming" the younger QB is one of the most annoying football narratives out there, because it doesn't work that way. All of the supposedly successful stories are myths in that the veteran (Joe Montana, Favre, Warner) had very little interaction with the younger one (Young, Rodgers, Eli Manning) and sometimes the relationship was icy. At most, the vet gets vague credit for teaching the younger guy to "act like a pro."
At most, Fitz can just make Rosen feel comfortable and maybe get Rosen to be less of weirdo if those rumors are true. But even that might not be necessary as Philip Rivers obviously developed a personality completely different from his supposed mentor.