Can you tell me what number of grams of sugar should a 15 year old boy take in a day?

Depends on activity: dietary sugars should generally come from food, and be naturally occurring, as opposed to being added sugars. An active 15 yr old boy running track/cross country 3 seasons per year could need 4500 calories per day. A sedentary boy could need 2200 or fewer if he is overweight. Optimize vegetables, fruits, lean proteins, healthy fats, plenty of water in diet. might not need to count calories then.