What is the recommended daily Kj for a 13 year old boy?

It depends on how tall you are or how large you body frame is. Even the amount of exercise impacts on the total number of kilojules needed for 13 yr olds. As a gemeral guide somewhere between 7000-8000 KJ would be plenty. But keep in mind that 13 yr old girls are going through puberty at this stage and would need extra KJs for that growth.