From what I've read: "Just hours after health officials in LA County announced the boy had died from the coronavirus — health officials said there may be an “alternate explanation” and that his death would be further investigated."
I think maybe because of his age, his lack of insurance and subsequent directive to go to the hospital rather than urgent care because of his lack of insurance they are probably wanting to be a bit more careful in the investigation before definitively ruling his death primarily caused by complications of coronavirus.