Don't know about vans but had a similar experience when tried to put daughter on our current insurance with mainstream company, basically priced because they don't want the business. Cancelled policy and insured with Diamond (found via comparethemarket) at much lower but still somewhat eye-watering price. Renewed twice with them and then went to LV this year who are now substantially cheaper (age 20, 3 years since test).
Depending on your situation in terms of son needing to drive the van all the time its worth checking costs for policy alteration, if not needing year round access the cheapest option is to insure for full year and then remove driver when you don't need and take the refund .