You are absolutely right that aspiring won't do anything for a heart valve problem.
A purple comb suggests not enough oxygen to the periphery which implies more of the heart valve type problem rather than narrowing arteries to the heart (which is what causes a heart attack and could be improved with aspirin if chickens have the same physiology as people in that department).
Does he have any other signs of low oxygenation? Shortness of breath? Does he have any discernable fluid build-up in his abdomen, chest or feet?
Of course low oxygen to the periphery could also happen because of respiratory issues, but my guess is you have ruled that out from observation.
Good luck with him. You can only do what you can do.