No National Insurance is payable by under-16s, so he won't need an NI number (and it will usually be issued automatically anyway at the appropriate time).
Most under-16s don't earn enough to breach the earnings threshold, but as he is working in the holidays, he might. Nevertheless, he won't have to pay NI (although he would have to pay Income Tax in the unlikely event his annual earnings exceeded �5439).
If the company does not have experience of employing under-16s, you may have to ask them to call HMRC to seek clarification, as they will likely be used to insisting onn an NI number as a matter of course.