I'm so sorry to hear about your kitty's HCM diagnosis. Has he had an cardiac ultrasound to determine the severity of the disease or is he taking any meds?
I have a 4 year old male cat, Peter, who was diagnosed with mild HCM 2 1/2 years ago, after our vet detected a grade 2 heart murmur on two occasions and xrays showed his heart is slightly enlarged. Since then he's been on a low dosage of atenolol daily to regulate his blood pressure and prevent blood clots, along with taking 1/2 baby aspirin twice a week to keep his blood thin. For now, he's going back for annual ultrasounds. The meds seem to being doing the trick because his last ultrasound showed no further progression of the disease. He's due for another ultrasound next month.
Of course our situation's different since Pete's a young guy and the disease was caught early. My advice would be to consult with your vet and feline cardiologist if you haven't already on what you can expect. You may also want to check out this online support group for folks whose cats have heart disease. I've found it to be very helpful:http://pets.groups.yahoo.com/group/feline-heart/
Good luck with your sweet boy. Feel free to PM me if you need anything at all.