is it at all possible for u to be a bit shifty and tell him the pup had to b put down and offer him a full refund? then care for the puppy yourself and sell it again once its healthy enough?
he doesnt sound like he's taking the puppy's welfare seriously at all, poor little bugger
That's exactly what I would do.

Every time we've bought a puppy the breeders have made sure we would be suitable owners before agreeing to sell it to us. I still keep in contact with the breeder of my dog, I send her photos and have even taken Monti to her house for a play with her mum lol.