First of all, you're absolutely allowed to say no to someone without having to justify yourself.
Secondly, the people saying if he's working he has to be over 16 are totally wrong. The minimum age you have to be to work is 13, but it can only be part time. You can only work full time from 16 years old.
Judging off the fact you're both still at school, it would be impossible for him to work full time.