Co-enzyme Q10 (co-Q10) is an excellent heart supplement -- especially for cardiomyopathy. Give approximately 3 times his weight 2-3 times a day. If he's 10 pounds then the dosage would be 30 mg 3 times a day. You can give more than that, it's just a baseline based on my experience with my cat and healing his cardiomyopathy. I gave him 30 mg twice a day and in 8 months, his heart had returned to normal size.
I also put him on a homemade diet. Proper nutrition is key. I'd also seriously consider what is in the water you're giving him. I'd switch to a RO bottled water and supplement with a pinch of RealSalt to replace the minerals.