In what way can a pure vegetarian teen maintain a healthy diet. meaning, what should he eat and not eat etc.?

Your choice?: If this is a lifestyle being forced on you by your parents, you are well within your rights to refuse. The health benefits have resisted scientific demonstration, but the moral statement is powerful. Focus on fruits, grains and vegetables, avoid empty calories / junk, get plenty of protein (soy's good), and you must supplement (iron, b12, vitamin d, iodine, others).