My DS was also diagnosed with a murmur at his 6 week check.
His pead said the same as everyone here - it's nothing serious because he is thriving, they just have a listen at every check up appointment to see if it's still there, and if he still has it at 1 year he can maybe go to a cardiologist but it probably won't be necessary.
He has his 4 month check on Wednesday so I'm curious as to whether he still has it.
I also have one that is only audible when I'm pregnant (to do with increased blood volume which makes it louder).
Try not stress your little man will be fine