OP, what’s done is done now but tbh it was unwise of him not to check what the price would be once he passed his test before he actually passed the test.
I’m guessing this is a change to an existing policy and not a brand new policy in which case he sadly won’t have a cooling off period, but if it is a brand new policy then he will.
I used to work for an insurance company (ironic given I don’t drive,) and we regularly had calls from people, parents usually, who wanted to know A, what kind of car it was best to drive, and B, what the premium would be.
The advice we always gave was that it would need to be a low grouped car, we used to recommend the Fiat Brava at the time but that was about 25 years ago.
But with regards to your question on why premiums can’t be capped, you need to look at not only what the person is paying, but what they’re paying for. So if a 19 year old crashes his car into another car, or causes a pile-up, or damage to other property, if people are killed or seriously injured, then his insurance is liable for all of that.
The insurance companies aren’t responsible for how much money their clients have. If they want to spend less then they have to buy a lower grouped car.But it seems your son didn’t even look into that when he bought his car.
And young males are by far the highest risk.
I remember one memorable call from a potential customer wanting advice as to what kind of car to buy her son. She said that he was one of a group of 5 who were gradually passing their tests, and thus far 4 of them had been bought new cars as birthday presents by their parents. In the last six months, he had been to three of their funerals. All because they’d crashed their cars.