did they not perform any tests? even a nuclear scan? Don't they know that a bypass can shut down after just 2 weeks? I would go back to a cardiologist and tell them you don't feel right and haven't felt right since surgery. Tell them surgery doesn't mean it's fixed, many things can go wrong. Then ask them to run more tests because you struggle so much in life.
I had a double by-pass 2 years ago. I still suffer from breathlessness. I spoke to 3 different cardiologists and they all say the same thing: "you just need to get back the in shape" but if is more than that. I often feel like I am suffocating from lack of oxygen. I sometimes need to take in a very deep breath just to be able to finish a sentence.
I would go see the Doctor again. Immediately after my bypass, I was not breathless. It is difficult to fully expand your lungs for months after bypass surgery but I don't know if this is the problem with his lungs? Have they indicated other flaws with his lungs?