We have a 5 week old pup with a possible heart defect. Should we wait for him to die naturally or should we put him down? What would you do?

Hi

We've been breeding dogs for about the past 6 years. In the last two litters how ever we've had one puppy with a heart defect. We breed two of our females with this male and the one female's pups have always been healthy so we figured the defect was coming through from the other female so we've stopped breeding her. A third female that would never mate with our male finally became pregnant and had a litter almost 5 weeks ago now through c-section. Two of her pups did not make it. So we have the same father but a different female completely. We've been bottle feeding this pup because his mother was not able to nurse. He eats well, is fat and very strong. At almost 5 weeks old, this pup we think possibly might have the same heart defect. How do we find out for sure where the defect is coming from? This puppy's mom is a toy and I've been reading where these defects can be common in toy breeds. I'm breeding shitzu/poodle cross puppies and cockapoo/shitzu cross puppies. Other than surgery do you know of any other means of helping this puppy make it if he does have the defect. His gums are pale and at times his heart seems to beat really hard. Is it cruel to keep the puppy at home, comfortable as possible and let him die naturally? We had our other one put down and I just felt awful doing that.

Thank you,
Donna