HCM in kids is sort of unusual, but certainly not unheard of. Is your child being treated at Mayo Clinic? They have among the best doctors for HCM in the world. The most important thing with HCM is to find a doctor who has experience with HCM. This is really critical. HCM is now a very treatable disease. But the first thing you need to know is whether your kid would benefit from an ICD and the second thing to know is whether he is obstructed.
Does anyone else in your family have HCM? Do you have other kids? If so, genetic testing might also be indicated.