The thing is that similarly to human insurance, if you apply for a new policy you will not be covered for anything he has suffered from in the past.
I suppose the insurers say to themselves, "The dog must have had a lot of things wrong with him in seven years, so there will be plenty of things that we won't have to pay out for."
If your dog has had a fairly healthy life, there is nothing to stop you asking for a reduction to the same level they are offering for a new customer.