A man in my ward was getting home from work and pulling into the driveway. His family was leaving for Disneyland as soon as he got home. One set of grandparents were going with them. He was looking at his boys play basketball while he pulled in. Unbeknownst to him his three year old daughter was running toward the driveway to greet her dad. She tripped and landed in front of the wheel. She was run over and killed. He, and his family, went from being excited to go to Disneyland to grieving over the death of their youngest sister.
During the viewing on the morning of the funeral, the father couldn't deal with his emotions and walked out of the room. The Bishop was consoling him when they were both shocked to find President Monson standing next to them. President Monson asked if he was the father, to which he answered yes. President Monson's first words to him were, "Don't blame yourself." I wasn't right there to hear what he said personally, but I was the executive secretary at the time and heard the Bishop's first-hand account. President Monson stayed and spoke at the funeral service.
After the funeral everyone was curious to know what connection this grieving family had with President Monson. Come to find out there wasn't any. Apparently President Monson either saw personally or was told about the event as described in the child's obituary. While President Monson was there, someone asked him why he came. His response was that he knew where he was needed. He drove himself, no security or anything.
While President Monson can't be to all funerals which involve this kind or tragedy I'm sure, but I can't help but think he would say the same to you if he were to be able to meet you in person. I'm sure you know in your heart that you've analyzed the situation a million times. That you've considered over and over again what you could have done to prevent this tragedy. The thing is that it was not your intent to harm your beautiful son. That it was indeed a pure accident. I know there are many times in my life where I or a member of my family would have died if not for either blind luck or divine intervention. And when I hear about situations like yours where the accident was not averted, my heart goes out to all involved.
I have never had a family member die unexpectedly, so I have nothing to compare to as to your feelings now. All I can say is that you'll surely remember this incident for the rest of your life, but it doesn't have to be such a bitter pill to swallow for all that time. Easy for me to say, I know, but I wish you and your family feel the love of the Savior as deeply as he wishes you to. You and your family will go through the stages of grief, but may the thought that it wasn't your fault sink into your soul and give you comfort and peace for the rest of your days.