I don't think you'll find anywhere that covers him for a pre-existing condition; that was one of the problems we had with covering our cat. We had insurance with PetPlan for 9 years for her, and they wouldn't pay out on anything because it all got blamed on a pre-existing condition.
Marks and Spencer cover older animals, but they don't cover the pre-existing conditions.
I'm afraid you are very unlikely to get any cover now that is worthwhile. You certainly won't get anyone to cover him for existing conditions, and the premiums at his age are going to be pretty high anyway.
All I can suggest is that you open a building soc. account with a small lump sum (say £200), and put in say £30, use that to pay medical bills for him.