Competitive bodybuilding (reducing bodyfat as low as possible while maintaining high muscle mass for posing and judging) is not usually appropriate for most adolescents, especially those under the age of 16, but the discipline of bodybuilding (building muscle and minimizing or reducing bodyfat) can be practiced by adolescents. Adolescents boys are growing at a very fast rate and require large amounts of calories and nutrients to accomadate that growth regardless of any bodybuilding goals. Adolescents also have extremely high metabolisms so they can handle and require more calories than most older inviduals.If an adolescent is engaged in a lot of physical activities or sports then he will need larger amounts of calories for energy and protein for muscle recovery. For an adolescent or anyone else to put on additional muscle, they must consume even more calories and protein.
Adolescents looking to engage in bodybuilding will have somewhat similar nutritional requirements as any bodybuilder would. The best nutrition plan is usually to eat 5-6 smaller meals throughout the day with a Breakfast-Snack-Lunch-Snack-Dinner-Possible 3rd Snack structure instead of the traditionally larger "3 square meals a day". Eating every 3 hours or so will ensure that muscles are constantly being fueled with protein and carbohydrates, metabolism stays up to continue burning fat, and blood sugar does not get too low which compromises energy levels and workout performance. Caloric intake should be "frontloaded" so more food is eaten in the first half of the day when metabolism is higher than eaten in the second half of the day when metabolism is significantly lower. A healthy snack can be eaten after dinner but simple sugars should be avoided late in the day as they spike blood sugar and insulin levels which lead to increased storage of bodyfat. Small to moderate amounts of lean proteins should be present in all meals and snacks for a constant supply of amino acids (building blocks of protein and muscle). Excellent examples of this are low-fat or fat free dairy products, (grilled) chicken breast, turkey, tuna, salmon, soy, and lean cuts of red meat. The most important time to consume protein is post-workout so that the muscles broken down during exercise will be provided the raw materials to rebuild bigger and stronger.
Dairy products are also crucial for adolescents because they provide the Calcium and Vitamin D which is critical for the major bone growth they experience. The only nutritional supplements I would recommend for adolescent bodybuilders would be a multivitamin and possibly a scoop (about 20 grams) of whey protein powder per day, both of which would ensure they get at least the minimal amounts of these crucial nutrients.