What two things are influencing Colman's behavior (his willingness to tell personal things about his family)?

I suppose his father's sexuality as well as his own insecurities around masculinity and identity are at play. Colman finds it especially hard to accept that his father was biologically female. He finds this out when he is thirty, and feels resentment towards Joss, whom he saw as always strict with him. As a child, Colman was naughty and difficult to deal with, and as an adult he still exhibits a child-like vulnerability. He is both confused by and angry with Joss, and tries to prove his own masculinity to himself and Sophie. He initially decides to cooperate with Sophie in telling Joss's story, but over time his love for his father trumps his anger and he pulls back on cooperation. At the end of the novel, bolstered by his visit with his grandmother, he reads his father's last letter to him and joins up with his mother in Torr.