My first boxer and one of my current boxers both had heart mummers.
First of all the ER vet cannot grade the mummer just listening to his heart on a once off. If the dog is nervous/stressed its going to effect what they are hearing. To really know the grade of your boxers mummer and the cause of it he needs to be examined by a cardiologist. So please don't let what the ER vet said scare you.
Hooligan our first boxer had a grade 1-2 his whole life, caused by sub aortic stenosis, never affected him, we lost him to cancer not the heart.
Finnegan my white male now 3 years old also has a grade 2 mummer caused by Mitral Valve Regurgitation, so far it does not effect him but we will continue to monitor on an annual basis.
Dogs can live long healthy lives with mummers, and a cardiologist is the only one who can tell you exactly what you are dealing with.
Good luck and let us know how you make out
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