This is called cardiac cachexia, and you can google and read about it. There is really not much that can be done. I'd just be happy that his appetite is still good.

I absolutely would not feed anything extra like the Satin Balls - way too high in sodium and way too high in phosphorus and high fat also. Most dogs in advanced heart failure have marginal kidney functioning and I sure wouldn't feed a high phosphorus food. Managing heart failure and concurrent kidney failure is no fun - I've been there. I tried to add good quality protein with lower phosphorus amounts for my geriatric dogs with heart disease - often cooked chicken thigh and/or fish. This would be added to the regular high quality commercial food.

I have friends with geriatric dogs who are using water treadmill therapy to add muscle mass and strengthen rear legs - but these dogs are not in heart failure. I'd want to speak with the cardiologist before starting any exercise program like that.