M.P.
D., I am so sorry that this has happened to you. I have dealt with family death's several times. Twice with children involved. I think the best way to handle it is to be open (honest) with your son. Tell him, briefly what happened. I think it's appropriate to include that this happened because the other driver was drunk. But don't get caught up in the anger at the drunk driver. Focus on and share your son's emotions. This does include anger at the drunk but the focus needs to be on the feelings about losing his grandpa.
It may be difficult to know the difference in focus. Perhaps this will help. Often people get so focused on the anger that they don't acknowledge the sorrow. Both are important.
There are several children's books that are about grief. You can find them at the library. One that I like is Freddy the Leaf by Leo Buscalia. I'm not sure that's the correct spelling of his last name. And there's one that is specifically about the death of a grandparent that I read to my granddaughter, Monet, whose other grandma died when she was 5.
This will be such a difficult time for both of you. Be kind to yourself. Remember to focus on your own grief with an adult. This is especially true if you have unresolved issues with your father. Share your sorrow with your son and encourage him to share his with you. Children's feelings are simpler and in the present because they don't have the long years of history that adults have. If you are overwhelmed with grief and anger find someone that he's close to to help you talk with him.
Reassure him that you will always take care of him. He may feel that since his grandpa was killed you might be too. Now is the time for a positive focus on living especially on the fact that you will be alive and will take care of him.
My granddaughter has been concerned about my death since her other grandma died last year. Sometimes I have said that I will some day die but I continue to focus on the fact that I will be alive many more years. When she says, "I want you to live as long as I live," I say I want that too. I do not remind her that it's unlikely. She is needing security not reality.
For you, if he asks, you could say that his grandfather was killed in an accident and then reassure him that you're going to continue taking care of him. You can remind him of the reality that it is very unlikely that you will die. Focus on giving him a sense of security.
I recommend getting involved in a grief support group. The ones I know of for adults are provided by hospitals. There is a good one for kids called Me Too, and Company. Their Portland numbers are ###-###-#### and ###-###-####.
I also think it's usually good for kids to go to the funeral as long as there will be someone to be with them throughout the entire time who is also able to respond to the child's feelings. This may be someone other than yourself because you may be too overwhelmed by your own feelings to help him with his. And you will also be responding to others.
My granddaughter attended my mother's funeral when she was 2 1/2. Most family members were glad to see her there. I think that she was a symbol of the joy that will continue. She was certainly a comfort to me as I held her. Of course, because of her age, being there had no meaning for her. Because my brother objected she did not go to her grandfather's funeral which was a couple of weeks later.
Going to the funeral may be helpful for your son. It is a ritual during which to remember and to say goodbye to his grandpa. Whether or not he will benefit from this ritual depends on his own personality. It is also important that his presence be accepted by others and that he has his own support. Attending is an individual family decision made by adults.
You can also make your own ritual to say goodbye. My friend, whose husband just died, put a picture of him and several things related to his living on a table. She "talked" to him while she was looking at the table. Last week she watched the Oregon/Oregon State game in memory of him. He was an Oregon State alumn and always watched the game. You could do the same sort of things with your son.
Talk about your father and his grandfather, remembering the good times too. Death feels like the end of a relationship but it's not. He will always be alive in your's and your son's mind and soul.
My heart goes out to you.