WolfLady going to the vet I work at was my second opinion. I had already taken him to a vet which is how I discovered he had the grd 3. They mark it on a scale of up to 6.
After hearing you say that your vet said it was nothing to worry about it makes me think if I gave him back it would be for no reason and that I would be wrong.
How he acts is kind of weird he has this odd routine it seems. One day he will just sleep all day and not move from that spot all day, not even to eat or drink and usually the next day he is more active (active as in walking around the house and eating/drinking). Sometimes though he stays sleeping for 2 days.

The only thing I'd worry about with sending him back to his breeder is that he would have to fly and he's not UTD in shots because of his stupid previous owner. I wouldn't want to put more stress on him with his heart because with high murrmors they can have heart attacks more easily.