Hi mate,
I'm no expert with the privisional and younger driver insurance but I would assume that insurance companies would not see the provisional driving as actual driving experience and that would lead me to think that your son would not gain no claims bonus. He is being 100% assisted in driving when on a provisional, the rise in premium is because of that, it is a lot at first but it drops dramatically after the first few years... I had a car worth £300 when I passed 5-6 years ago and was paying 350 a month for insurance, go figure... haha
What car is he driving? I know that some insurance companies bring the premium down if he is willing to put a 'black box' in the car, if he drives the vehicle to their standard after a certain period of time then he may come across a reduction in premium also. The threat of that of course, being new to driving and having that freedom, he may not drive exactly how he learnt to drive...
Hope you get it sorted either way but I can only imagine it is a case of 'biting the bullet'
TK