My son who’s 24 has 7 years NCB so has no problem getting insurance despite his age. I would say a lot of people on here are a much bigger risk than him.
VERY frustrating for him BUT as said before its all about stats and historically 25 years old has been the trigger point for a step change in car insurance. There will always be those under 25 who are the safest drivers in the world and of course quite a few older who will be terrible drivers. (Its all been said before but its just stats!). Fortunately your son only has a year to go at which point I imagine he will be very happy