A heart murmur is just an abnormal sound, it doesn't tell you much about what type of heart disease (if any) might be present. Having said that, if I were going to play the odds... with his breed/age/onset I would bet on something called mitral valve insufficiency. It really takes an ultrasound of the heart called an echocardiogram to diagnose the actual cause of the murmur, though.
IF it is diagnosed as mitral valve insufficiency, the easiest way to think of it is that one of the valves inside the heart gets a bit "leaky", creating some turbulent blood flow and backflow that we hear as a murmur. In the early stages it really is nothing to be concerned about, and a lot of dogs live with minor, stable valve disease that never progresses to
heart disease for years or even forever. I personally had a dog who developed this at maybe 7-8 years old and her murmur never progressed beyond a grade 1 or caused her any problems in her life. For other dogs the leaky valve becomes severe enough over time to eventually cause changes in the heart itself that can ultimately lead to congestive heart failure (fluid in the lungs).
But honestly once you figure out what is causing the murmur, IF turns out to be mitral valve disease I wouldn't lose one wink of sleep over it. There are tons of dogs walking around out there with mild mitral valve disease living their normal lives and doing all their normal things. With a stable grade 1 mitral murmur you probably won't need to do anything about it other than monitoring for changes. With a few exceptions (like cavaliers as a group, or some isolated individuals) any changes in the heart usually occur over a period of years and even congestive heart failure is usually manageable with medications for long periods of time.
Obviously if it turns out to be something else, that was a lot of useless words.