I'm about to insure my son for a new car, he has about 5 years experience as an "named driver" will that help to reduce the cost for his insurance significantly?

The difficulty is that insurers have no idea how much driving he has actually done in those 5 years. As it's NCB that is more highly prized, zero NCB will unfortunately narrow down the list of those prepared to quote at all.

Ok, it's a while since, but in similar circumstances I got a 2 years "introductory discount" (and a sensible base price) from my father's insurer, which they also treated as 3 years earned NCD come renewal. NB; YMMV.