It's completely impossible to predict. But it basically comes down to how often they get crashed and how much it costs insurers when it does. Some people I know have found Focus sized cars better for new drivers.
Often cars with a following (classic cars in waiting) like the Panda
@john_smith suggested can be surprisingly easy to insure.
Also who your son would have to be the main driver, having an lower risk driver as a additional driver will often lower the cost. I still had my dad on my insurance into my late 30s because it actually reduced my insurance cost, although by the I removed him from the policy it was only saving me £3 a year.