Hi S.,
Whatever you decide to tell your son, please don't talk about his father in a bad light. I would suggest limiting the explanation to a minimum at this point since he is too young to understand any detailed explanation. I would leave out the facts that you mentioned in your request such as his father left you when you were pregnant, you haven't heard from him since, the fact that you think he's a bad father, etc. These are issues between you and the father, and your son should not be burdened with it, especially at such a young age. Just remember that your son sees himself or will see himself as an extension of his father, so whatever you say about his father is a reflection upon himself. Please be gentle with what you eventually decide to tell him. Good luck!