Thanks for insurance advice! Our existing multi car deal (we have 3 cars already) is super-cheap and excludes under 25s and over 75s!
I went on MSE, read Martyn's advice and ended up with a great quote from Smart Wheels (MoreThan) who do a black box policy for new drivers. It's £980 a year when he's passed his test and includes loads of stuff. DS has his granny's 15yr old 1.3litre Polo and she and DH are named drivers (having an experienced named driver brings down the cost) but he's the main driver.
Interestingly, the "young driver" specialists wouldn't quote for him and one of the things was that the car is only worth £500 so I suppose they think he won't be bothered about crashing it. (It's got FSH and only 30000miles on the clock! But is just really old)
Anyway it's been a learning experience 
Fingers crosses he passes....