Another thing that you need to consider is that a diagnoses in childhood/teens has a different prognosis than if a person is diagnosed in adulthood. The reason for that is because the child/teen is still growing, so the heart muscles can really start growing at a faster pace due to the hormone changes occuring in the body.
Have you talked to anyone from the HCMA ? I would give them a try, In my opinion the best place to start. You are not alone there are many people going through what you are. I was dx with HOCM in 1985 in my mid twenties. I wish you all the best of luck !!
My daughter was diagnosed with a very severe form of HCM (all of her ventricular walls and both sides of her septal walls were 4cm thick and she had several electrical problems that were also going on: SSS, W-P-W, RBBB, LBBB and a Maheim Fiber as well) when she was 6 years old. I know what you are going through, believe me! We have been fortunate to have had very good medical care for her, it's probably what kept her alive until she had her heart transplant at the age of 22. The doctors can tell if your son has athletic heart easily enough; in this problem all of the walls of the heart are thickened due to being worked (such as in weight lifting) in HCM, only some of the walls, or perhaps only one wall, is thickened. That's how they know. Usually, a second echo is done if there is a question on the diagnoses, obviously, your doctor is pretty sure; echoes on my daughter were done every six months or so. 48-72 hour Holters should also be done to rule out any life threatening arrhythmias. One test you should be asking for is an EP Study which checks the electrical system of the heart. This disease can result in a sudden death event, like seen in basketball players who die suddenly on the game court. (sudden death is rare, only at 2%) Your son should not be in sports or in PE at school as sometimes the PE teachers can 'push' these kids too far; also, teen boys are too competitive for their own good. You asked how you cope as parents? This is a terminal illness and your son will eventually need a transplant to live, you need to understand that, believe it or not, it will help you to survive. People used to ask us that question all the time and my response was always the same: I used to tell them this: " I feel more sorry for the parents who's child goes out and is hit and killed by the school bus on his way to school; those parents never saw it coming, at least we know our daughter has this problem and could die suddenly." You are not headed down an easy road, but remember that God will give you and your husband a strength that few parents will ever know and you all will grow from what happens on your journey.