Why doesn’t finny believe Gene’s confession? Call him?

Gene doesn't actually confess himself in this chapter. Brinker busts into their room in the morning, shocked to see Finny there; he uses the opportunity to reintroduce insinuations about Gene causing Finny's accident, but Finny doesn't want to think about it and deliberately doesn't take the hints. Gene brushes the uncomfortable situation aside by talking about enlisting, which Brinker is absolutely gung-ho about. Finny is not pleased at all with the idea that Gene could leave; he doesn't want Gene to do it, and makes this clear to Gene. Finny, at this point, probably knows the truth but has somehow justified it in his own mind in order to reclaim his friendship with Gene.