We had a puppy that had a grade 1 heart murmur at his first check up but by 5 months it had gone.
Mocca my little black & tan came to me at 7 months and she had a grade 1 heat murmur which caused her no problems she was always very active and only had to have medication as it progressed to a grade 4 at the age of 14.
I really wouldn't worry too much and wait and see how he goes, regular checks just to see if there is any change will keep your mind at rest.
