Car insurance renewal 300% difference in quotes.

My 18 year old son will have 1 years no claims next month on his 1.25 fiesta.
He will not know until he receives his A level results next month if he will carry on being a student or go into full time employment with a job offer he has received.
The insurance quotes for a student with many insurance companies including Admiral are coming out around the £800 mark.
If he goes into full time employment and enters a job title the insurance shoots us to around £2400.
I though student car insurance was more expensive but these quotes contradict this and a 300% increase is ridiculous.
Has anyone else has this issue and found a legal way around it or have an explanation for the premium difference.

Thanks in advance.