What you described sounds like how my Drake was acting when he had a Staph infection in his ears. I had to treat him with Baytril form the Vet's. Definitely keep an eye out if anyone else starts acting lethargic like he was. check all over the body for any infected-looking cuts or scrapes.
The Staph ear infection isn't common at all but it was very obvious with my boy(his sister also got it). Created a large pale yellow protruding plug in his ear hole that i had to remove. And of course Staph smells BAD.
Sorry you lost your boy, hopefully it was an isolated thing.