My son, who's four, told me that his father left him home alone. I don't know when this happened, and all I have is the child's word that it did happen (I believe him, but I'm fully aware that a 4-year-old's word carries very little weight). I don't want to confront my ex with this, as I know from past experience that confronting him only causes quite a bit of drama with little to no results. Plus he told our son not to tell me, and I'd prefer that he not find out that our son doesn't keep his secrets. What are my options here?

We live in South Dakota.

That doesn't sound good at all. Maybe you can say to him "so what did you and my son do last night" see what he replies. And keep an eye out for both of them.. A 4 year old should not be left alone under any circumstances. Hope this helped :/