He has hypertrophic cardiomyopathy.
In other words he has thicker than normal heart walls which make it harder for the blood to flow through his heart. It also means that it can swirl about instead of flowing easily through his left ventricle. But at the moment he’s OK. Just got to watch out for deterioration. He’s not showing any signs and he is a young cat.
I was so worried. He will be looked after. No meds required yet.