So sorry about your poor pup
What state did you buy him in? If it is in one with a puppy lemon law you may have some recourse.
So very sorry,hugs for your family
I am so sorry about your pup, but I know you made the right decision. Give all those other pups and extra hug tonight.
As sad as this is, remember the positive. The vet reminded me of these two points when I put my Mr Winston down back in September:
(modified slightly to fit your situation)
1) Since you are dog lovers, you gave this little guy a good, loving home for the time he was with you. We should all be so lucky to be in a place like that before our deaths. Plus I presume you were there with him at the end so he wasn't alone.
2) There are lots of good, friendly, wonderful dogs that are in dog shelters who are put down every day. Plus there are new pups being born everyday that will need a good home. By letting Duke go, you freed up resources to adopt another dog or pup, from a shelter or a litter. Maybe not soon but possibly one day.
It is hard even when you know it is the best thing to do.
He sure was cute.