Where can I go for cheap car insurance for a young male driver?

My 17 year old is taking lessons but we've discovered that the cost of adding him to our car insurance is prohibitive. I'd really like him to be able to drive our car as my husband is disabled and I also look after my 80 year old mother so having somebody else in the family with a licence will be a real boon, but the current quotes for adding him to my insurance seem to be pushing the cost up from £260 per year to £250 per month!