Aw I know how you feel, my son is grown now but I was in exactly the same position when he was little. Agree with everyone just be honest with him. I was with my son and since he was old enough to really understand there is no blame and a lot of respect. Especially as his dad was often letting him down as well. Its hard not to show you are angry and upset about that, but kids aren't daft they see how things are. He may not really understand all of it now but he will, and will appreciate you telling him rather than him wonder what he might have done, when it's nothing! :)