You will get lots of different opinions on this.
First of all - he needs to find out if flying is really for him.
That means a few hours instruction at the local flight school.
To be honest, there is no real point in getting him started on a PPL at 15.
My advice would be to keep him in school but keep him focused with the odd hour up in the air.
You would be amazed the amount of kids who actually get into a plane and realise that it is not what they imagined it would be.
After his schooling, then you can start looking at doing things step-by-step.
But remember: it all initially boils down to being able to get a Class 1 medical.
If he can't get that......the dream could well be over.
Farrell