Help with Insuring my Son's '68 Dart

I have gotten a quote from Grundy, but they are prohibitively expensive for the other 2 vehicles we have to bring over to get them to cover the Dart (more than 2x State Farm's rates).
You have two other classics insured with State Farm? Those aren't going to be agreed value either. Why can't you just insure the one car with Grundy? Is his age a factor? A lot of classic car insurance companies won't insure a driver under 25. They also require the car to be kept in a locked garage. I think you are SOL.