XMc - What about putting your kitty back on food he likes, and the baby asprin, then see how he's doing in a month? Hold off on the ultrasound for now.

Not sure about kitties, but I had a dog diagnosed with a heart murmur around age 4 or 5 that her vet never suggested treatment because otherwise she was healthy. She had a good diet, silky coat, played hard without shortness of breath, and went for regular walks. It wasn't until the winter of her life, at age 15 that she started coughing and having the shortness of breath I was warned about, that she was finally put on heart meds.

Good luck. Your kitty is young, which is on your side. With food he likes, he might turn around. Or even some nice treats?