Everyone has his or her own normal biological time table for pubertal development. Some boys, for example, show signs of puberty while still in elementary school, while others can get well into high school still looking like little boys.
Being different from the average can be especially hard on a boy who's puberty is delayed. If your son is in this situation, there are several things you can do to help. As a physician, I've given temporary gym excuses to boys who were too embarrassed by lack of pubic hair, or genital growth to use showers after gym. Six months later, development may be sufficient to allow the boy to shower with peers, without undue embarrassment.
Since late development is often a hereditary characteristic, it may help to share this fact with your son, especially if the father, or another male relative happened to be a late bloomer. This can help reassure the boy that he is normal -- that he will develop into a man in his own time and help him to feel that someone understands.