You will need to be sitting down when you get the quotes! Young males are very expensive to insure. Sometimes adding an older named driver to the policy can help a bit. CIS (the Co-op) are the ones I'd recommend.
Good luck.
Have you and he considered a pass plus course? It's a series of lessons, in modules, designed to improve the standard of young drivers by concentrating on areas which, the designers believe, are not covered by the standard driving test. My daughter, then 17, took the course and found it helpful plus it got her a discount on her insurance.