New driver short term insurance

We'd like to find occasional insurance to enable our son to use our car for odd days here and there. He lives in Cambridge, we in Lancaster, so most of the time we can't give him chance to keep his hand in with driving but we'd like to take out short-term insurance for his use of our car when possible. He doesn't have a car and won't have for the foreseeable future. He's 24 - 25 in October - and passed his test only in August. Companies I've looked at on the web insist on 12 months full license minimum before they'll give such cover. Any suggestions, please?