I'd certainly start by taking him to a cardiologist rather than a vet so that you can get the best information possible on what is going on and what the options are. If he has the murmur Nicki mentions it would be shame not to try and seek whether there are possible sources of funding as with one surgical procedure that can even be done simply by keyhole, he would likely live a normal lifespan. It doesn't sound as if he has a great breeder but if she would take him back and do the surgery, maybe as painful as it is, that would be an option to consider to give him a chance of a full life. I kind of doubt she would though.
Maybe let us know generally where you are (ie what country) and people may have some suggestions.