If he's in good health I'd try and get hold of his records and fax them to insurance company. I think they do tend to give higher quotes with older dogs anyway, but more so if they have previously been uninsured as some owners may just insure the dog as they suspect a problem - it would be worth giving them a phone and explaining your situation. They may also just give high quots to discourage people using them, as they hav to pay out more for older dogs.
Also, it may work out better for you just putting the money into a bank account every month , and using that instead of paying for insurance, if that proves too costly. I know some people who feel this is a better idea than insurance.