It will depend on the flexibility of the driving school.
Driving instructors are obliged to check that their pupils have a valid driving licence before permitting them to drive. Your son should phone the school, as soon as possible, to see what they say. (Unfortunately, if it's a big firm, their office might not open until Tuesday but you could probably contact an independent instructor, who works from home, over the holiday weekend).
I suggest that your son should offer to present the paper part of the licence (which proves that ' Mr Son-of-Shazza' is licensed to drive), together with his passport (which proves that he is 'Mr Son-of-Shazza'). Hopefully, common sense will prevail and he'll be able to take his first lesson.
Chris