My son (13 year old) is training very hard for swimming nationals 4 hrs a day. how can we supplement his diet. what should he eat?

Balance is important: Vitamins (esp Vitamin D), minerals, hydration, lean protein (not high quantity), simple carbs (pretzels, honey, fruits, sport drinks) directly before, during and directly after workouts, complex carbs (legumes, fruits, vegetables) outside of workouts, and a ratio of 3-4:1 carbs to protein. Best regards for good health and sports success!