Oh Jo I'm really sorry to hear about Seth - I know how much you were looking forward to working him. It seems that there is no guarantee for even the most well bred dogs.
My own dog trainer used to breed working labs and her last litter had parents who had excellent hip scores and yet one of the litter developed HD. Thankfully he has pretty normal life after courses of hydrotherapy, after being diagnosed at about the same age so there is some hope.