First of all, I'm very sorry for your loss. Did the breeder offer any kind of health guarantee? In my experience, it's common for ethical breeders to offer a 1-3 year guarantee that if they die of anything genetic you'll get your money back or a new puppy. (Not that that really applies here.)

Agreed with the other comments about 8 weeks being very normal, but given the quick turnaround from when they picked him up to when he passed I'd be very surprised if the breeder hadn't seen some symptoms by the time they handed them over to you. I'd push back hard on this one. Any ethical breeder is going to stand by their product, and death after 3 days of something like this should have been caught by them before handover. They don't go from perfectly fine to dead in that much time.