If you refrigerated his body, you can send it off to a state veterinary lab to have them check what he died from. You would send it over ice (Not frozen), they have info on the state lab website. If you search this site, there's the link somewhere (sorry I don't know it) to look up the closest lab to you.
If you feel up to it, you can try and perform a home autopsy, and take lots of pics, and folks on here might be able to help you. But unless it's obvious, there's only so much one can tell visually.
Do you feed layer feed to your entire flock, or do you feed an all-flock feed with oyster shell/calcium and grit on the side? Sometimes, if you feed layer to everyone, the roosters can have kidney failure sometime around 4-5 years because of the higher calcium content in the layer feed. Not all roosters have this problem, but some do.
So sorry you lost your rooster. That is hard. Could have been any of the causes you list.
