How to Explain the Death of a Loved One to Your Child on the Autism Spectrum

“When should I begin to talk with my grandchild about his grandfather’s (papa’s) sickness that will result in death? How best to approach the subject? Thank you for your assistance.”
 
The answer to your question would be “the sooner the better.” Kids, even those on the autism spectrum, typically know more than their parents and grandparents think they do. 
 
You can gauge what your grandson knows through the questions he asks. If he asks, for example, "Is grandpa going to die?" …he may not want to hear, "Everyone is going to die someday." Instead, this can be a signal that he knows grandpa’s condition is life-threatening.

I recommend open and direct communication at all times. If you avoid your grandson’s questions, he may ask someone else or hold the questions in, which could result in unnecessary anxiety. Acknowledging rather than disregarding questions can build trust and show him that his concerns are important. This may increase the likelihood he will come to you with future questions.

So, be honest and concrete in discussions about death and dying. Avoid euphemisms. We use euphemisms to avoid uncomfortable subjects, but kids with Asperger's and High-Functioning Autism (who think very literally) may not pick up on these cues. 
 
For example, if you tell your grandson (after his grandfather’s death) that “grandpa is sleeping,” he may expect grandpa to wake up. If you then say that “grandpa can’t wake up,” your grandson may fear going to sleep and not waking up (you get the point).

Though the words are difficult to say, use terms like "die," "dead," and "dying." Also, considering finding books on the subject of “death of a loved one” or create some social stories around grief.