That's strange, because my heart cat Marius has a grade 3 heart murmur, and my vet indicated that it's really nothing to worry about at this point. Marius never had a heart murmur before, so having it appear suddenly (about 3 years ago now) was alarming. They did an ultrasound on him and discovered a grade 3. He doesn't require meds at this point, but I do have to keep a close eye to see if any of his behavior changes. My vet told me that as long as he's acting normal, that there's nothing to worry about. I'll just have to bring him in once a year or so to make sure it's not advancing. That was 3 years ago, and he's still consistent, which is good.
I would ask for a second opinion? How is Ivy acting? Is he behaving differently/more lethargic? It seems to me that they're treating this grade 3 as a grade 5 or 6, but maybe they use a different scale?

Regarding what to do with him if this really becomes serious, it probably would be in his best interest to go back to the breeder. Good luck and let us know what happens!