Your description of the problem existing on the right ventrical and existence of arrythmias sounds to me that he has arrythmogenic right ventricular cardiomyopathy (ARVC). Otherwise commonly known as, Boxer Cardiomyopthy.
There are a small subset of boxers who reach a stage of ARVC where the ventrical will dilate, which can sometimes cause a misdiagnosis. What is important to this case though is the arrythmia. DCM is a systemic heart problem, while ARVC is a electrical conduction problem. They are quite different disease.
I'm curious, did your vet mention ARVC? If so, did he mention why he didn't think that was it?
Either way, many a boxer with ARVC (and dogs with DCM) live a number a happy years with the proper treatment. While it can be heart breaking, you're one of the lucky ones who know about the issue and can treat it before it was too late.
Well I don't know the technical part and the vet did not mention ARVC. The only thing they did say was his heart was not contracting properly and that was what was going on. Would that make a difference in the medication or otherwise? How do they tell the difference anyway from DCM or the ARVC? They had a cardiologist review all the findings also. Thanks for the good information about both heart problems..