Please do tell the whole story. You have some great examples of the words to use, in the posts below.
One thing you may not have considered: Your two older kids might end up telling your youngest child what really happened. Or grandma or grandpa or an aunt or uncle or cousin will mention it sooner or later. jAnd your young son will be in the position then of saying, "But mom said....wait, was she telling the truth or are you?...."
Either your eight-year-old will sense that you are not telling him the full story, and he will ask his siblings or grandparents, or the siblings or grandparents or others will not realize you fudged the story with him and they'll somehow let the details out. It's also not fair for you to tell the older kids or grandparents, "Don't tell him the full story if he asks you" -- that puts them in a bind.. If you (and they) don't tell the full story, when your younger son eventually learns the truth -- and he will, sometime-- he will also realize that the people he loves don't tell him the full truth when bad things happen. That makes the bad thing worse for him, not better.
I know you feel you're protecting him from pain, but the larger issue is that one day he will find out the whole story. Wouldn't it be better for him to hear it up front from you, not in pieces or by accident from someone else?