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This is Dr. KV. I am sorry to hear the grave prognosis for your kitten. Congenital heart defects are very challenging to fix in tiny kittens. Giving him oxygen would be a temporary/palliative treatment, not a permanent solution. The only possible way to fix his heart would be surgery. If this is something you want to try, but your vet disagrees, the other option would be to have him see a cardiologist at a veterinary specialty center or veterinary school. They would be able to perform an echocardiogram to see if he would be a suitable canididate for surgery.
Most emergency veterinary hospitals have an oxygen chamber. The percent of oxygen delivered and humidity in the chamber must be carefully monitored. This is not something that can easily be done in a home environment since the kitten is so small. People can wear oxygen masks, but that would be stressful for the kitten...which could challenge his weak heart further.