My dog got Diagnosed with a heart murmur grade 3 hes a 6 month old boxer?

he never shows any fatigue he runs for ours and plays at the dog park everyday should i stop exercising him will it worsen his situation? they told me to take em back in 6 months an see how its the murmur is doing if it either disappeared or got worst and in that case they will set up at cardiac ultrasound and see if surgery if possible to better the situation should i wait that long? or should i get a second opinion?