How long does he need to stay alone? I don't want to depress him.
I would treat him like he's in quarantine. Any time a chicken dies, I treat it like they might have something that could be passed around, just in case. I might be doing overkill. Seramas seem to just drop dead from non-contagious things, too, so it could be nothing to worry about. I guess it just depends on what you feel comfortable with...