It sounds like there may be a couple of things going on with your son, although I can’t completely tell for sure with what is written here. I can try to give you some help with this, though.
As you know, a murmur is just an extra sound in the heart made by turbulent blood flow. That turbulence can be normal or abnormal. Abnormal means that there is something structurally wrong with the heart. Normal means that the blood is flowing through the heart normally. An innocent murmur (sometimes referred to as a benign, flow, or normal murmur) is just normal turbulence, and is seen in 2/3 of all kids. They have very specific characteristics in order to diagnose them, which hopefully your son’s primary care provider felt that he met.
However, he also has two other symptoms that may or may not be associated with his heart. The first is some sort of discomfort in his chest. With the information provided, it is difficult to say if it is benign, or not. It is reassuring to me that he says that he is “ok”, though, because kids (especially when they’re young) really will stop if something is hurting; they don’t “play through the pain” as adolescents and adults might. It may be that he is just aware of his heart beat, as many children are at this age, or he has some other chest wall discomfort. Your primary care provider, or the cardiologist in his upcoming appointment, can try to elicit more of a description from him, asking very specific questions about the quality, location, severity, duration, frequency, etc. that can help determine if this is truly cardiac, or not. Overall, the vast majority of chest discomfort in children is NOT cardiac in nature, which is definitely reassuring.
His other symptom of leg pain is also difficult to assess, only because I don’t have the same kind of information that I list above. The good news here is that leg pain associated with heart disease in children is VERY uncommon. The only time that we see that is if there is significant obstruction to blood flow in the aorta, the main artery carrying blood out to the body, in a condition called coarctation of the aorta. Most likely, your primary care provider has been doing good examinations of your son and has demonstrated that he has excellent blood flow to his legs, the lack of which would suggest a coarctation. Also, if his right arm blood pressure is normal, that would likely signify that he doesn’t have a diagnosis of coarctation. A much more common reason for leg pain in children is so-called growing pains, although I would need a fair bit more information to say for sure what is happening here.
Overall, the things that are typically most concerning with children at this age who have significant heart disease include getting out of breath with exercise sooner than peers, passing out with exercise or just after exercise, severe chest pain with exercise, or palpitations (feeling the heart skipping a beat or beating fast for no good reason while at rest). At any rate, it sounds like he’ll be undergoing a cardiology evaluation in the near future, so you’ll be able to get more complete answers to your concerns. The good news here is that the statistics are on your side.