Depending on the characteristics of the murmur, it is important that he have an echocardiogram to evaluate the cause of the murmur. The main concerns is hypertrophic cardiomyopathy. Not everyone with a murmur has heart disease, in fact it is pretty rare, but it is important to evaluate. I would recommend refraining from exercise until he has a cardiac evaluation. Over all his risk is probably low, but there is no reason to take any chances. I don't know the numbers on how common murmurs are but I would guess greater than 5%. It depends on what the murmur sounds like and that is where experience matters.
You should not feel badly. If you took your child to a doctor for every comment they made you would live in a doctors office. Being a doctor, patient and parent is very humbling and we are all learning every day.
I hope this helps and good luck with the doctors visit.
I just wanted to thank you for the fast reply
your welcome. I hope all goes well.