I'm so sorry to hear it!!!!!!!!
A friend of mine lost a dog even younger than yours. He wasn't even a year old yet, and seemed fine until about 9 months old when he started going downhill very rapidly. I wish so much that I could say something to cheer you up, but in my friend's case, there was nothing that could be done short of a heart transplant. My friend's dad is a renowned vet with lots of connections, but it didn't matter. :( :( :(
I hate to be so grim but I wrote this so you can prepare yourself for your visit to the cardiac specialist.
In humans, it can be a highly treatable condition. Perhaps in boxers it can be managed as well, if it's not severe. I'll keep you in my thoughts and prayers, and hope that your case is a mild, treatable one....
Take care.
-Linda