I understand being worried but it sounds like the vet thinks it will resolve as your pup grows. Even if it doesn't, my dog had a severe heart murmur (grade 5 or 6 I think) that required surgery to put a coil in. Seven years later she is still super fit and healthy and active - never would know she had heart surgery
So, even if it did get worse - which it sounds like it won't - your pup can still have a long and very active life. Hayden is half whippet and in spite of that coil races around like a speeding bullet!