I'm very sorry, it does sound like he has passed away. They can't breathe when on their backs, so they typically struggle when you flip them over. And the warm water would at least brighten his color up if he was alive. If he stayed black in the warm water then he's passed away. I agree that it's pretty apparent he died just from the photo of his eyes, but you always want to be 100% sure. I'm so sorry, that must be quite a shock when it happens so suddenly. I will say that in the photo from 4 days ago he looked very healthy, a good weight, good color, no signs of bone density issues or anything else.
I wish we could give you some explanation as to why he passed away, but there's just no way to know unless you take him to an experienced reptile vet to have a necropsy done. They don't usually cost much, and if nothing else you'd know why he died. My guess would be that he had some underlying health problem that hadn't shown any outward signs yet, like a cancer or liver disease, kidney disease, etc. Either way, I'm very, very sorry.
Not to be cold to you at all, but I just wanted to make sure that before you put any other reptile inside his enclosure or you use any of the stuff from inside his tank like rocks, branches, basking spots, hammocks, bowls, thermometers, anything at all that was inside his enclosure or even stuck to the outside or the lid, that you completely disinfect it all with either F10SC or bleach and hot water, let it all soak in it, scrub it all inside and out, and then rinse it all very well and dry it well. Just want to be careful in case it was some kind of parasite, bacteria, fungi, virus, etc. You just never know.