As he became ill after only five days then I would expect the breeder to accept responsibility (although to be fair they may not have known they had the problem). If your vet can write something to say that is his opinion, you could try sending the breeder a copy and saying you will expect them to meet any shortfall on the insurance, or else you will take them to the small claims court and report them to trading standards, as you have bought 'goods' that do not meet the standard required, i.e. living a normal healthy life as a puppy should.
I don't think the KC will be able to do anything, unless it can be proved that the breeder has broken any KC rules, which they probably have not done. Try contacting the breed club, and tell them what has happened. If the breeder is a club member, they may be able to help you to persuade the breeder to accept responsibility.
French Bulldog Club of England (Secretary) Mrs P Rankine-Parsons 01580 881043
Midland & Northern Counties French Bulldog Club (Secretary) Mr P Stemp 0208 348 2984.
None of this makes it any easier for you, and I truly hope the little chap gets over this 100%.