Half a seal bar? Ffs!
Teen boys need over 3000 calories a day depending on their age/size/growth/activities/etc. If you’ve got an active kid they will need even more!
My 13 year old daughter does about two hours of sport most days. She out eats her dad who is over six foot tall and works in a manual job. She wears a size 6-8 and is pure muscle.
Kids need to eat a lot when they are growing. Claire, if you read here, please let your son eat what he needs. I’m not saying let him eat processed crap all day long, but give good full meals and healthy snacks (toast and peanut butter, fruit, cheese and crackers, some eggs and toast, an omelette, handful of nuts and some berries, etc) when he needs them. If you control what your kid eats that he is already eating in his room or hiding food, you are setting him up for massive food issues and disordered eating. Teach him about foods that are good for your body and make you feel good and to eat when hungry and stop when not hungry any more.