Hi,
I’m so sorry for your sudden loss. There is no way to know what happened without a vet exam. A sudden death is often caused by an airborne toxin. Anything with a strong scent or fumes can be deadly to a bird, including air fresheners, like the plug in types, cleaning products, some non-stick cookware, candles and so many other products. For example if you decorated with something that is scented, that could be the cause. With a young bird, bacterial infections are very common and you may not have noticed he was sick. Sometimes it could be a genetic problem. It’s hard when there isn’t a clear answer, and a vet may or may not be able to determine the cause. I wish I could give you a reason this happened.
Take care,
Brenda