One way you know Stephen is growing into manhood, is his interactions with the other students. Their teasing now does not bother him, like it did before. Stephen also becomes aware of his family and their situation, he is no longer a little kid, who doesn't worry about his family's problems. And finally, i think his scene with the prostitute is the last sign that he has grown up.
Stpehen is not especially confident about woman, if you think about the tram ride home with the girl he likes he doesn't kiss her. He says he doesn't do so because he doesn't know if she wants him to. If he was more confident, he would just assume. ALthough he gets more confident, he still isn't sure of himself, except in situations like with the prostitute, where he knows they cannot say no.