When my grandmother died last year, we were very up front with our three year old. My original intent had been to keep him from the viewing and the funeral, but since my husband couldn't be there, I was there alone, and if I wanted to attend, I had to bring the boys. They did remarkably well. We told my three year old that she was old and her body stopped working--a variation would work for your father in law, too--he was getting older, and he got a very bad kind of sickness and it made his body weak until it didn't work anymore. My guess is that even if you're absolutely not religious, you might still believe in the concept of a soul, and we used our own religious beliefs to supplement that idea--grandma's soul still lives, in our hearts, in our memories, in heaven--but her body wasn't useful anymore, so they put it in the ground. He did go to the viewing, and when he saw her, I was very very thankful that most of the people there were relatives who love my son; he said, very loudly, Are those her bones in there?? A couple random things...I did have to work to keep my own grief in check. I'm not a hugely emotional person, but I did try to keep my tears more sedate. I also tried to keep my son from too close of contact with those who were really overcome (my dad, whose mother it was, really wanted my son close by, but he was really sobbing--I said no). We dealt with questions for probably 9 months about death. I would encourage you to do everything you can to indicate to him that this death is NOT commonplace, that it's NOT common, but if he insists that it can happen--which obviously it does--I just reassured my son that IF the worst were to happen, he still has tons of people who love him (My son is very, very good at taking a small idea and extrapolating. That might be more than a lot of 3 year olds want to know.). I would even perhaps allow him to visit his grandfather--to see the gradual death. My grandmother knew she was dying for quite a while, and it helped all of us to see her not be HER for a while before she passed on.Kids then see that death is usually not this sudden, I-lost-my-mother-last-night thing--he doesn't need to worry about YOU dying tonight. Yeah, they do recommend not telling your child the person went to sleep; then they fear they will die in their sleep. I just told my son her body stopped working, and when that happens, they die. We had a pretty good experience, all things considered. I'm so sorry for your loss; I hope you can find some peace for your son.