I'm the mother of a 9 month old, so I haven't been through this, but one thing you said did strike me as intersting.
You mentioned trying to explain and reason with your 5 year old, and you also seem to know reasoning with a 5 year old is near impossible. It occurred to me his behavior may become worse when you talk about the behavior to him and why it's not good. Depending on what you're telling him, he may realize that behavior has power over you and do it all the more to get his way. He probably understands enough to know it is threatening to you and he can therefore attempt to control you with it, however does not understand the full implications of just how much it really could hurt you if child services got involved. After all, he's only 5. lol. I'm not much experienced with 5 year olds, but they do seem old enough to be manipulative and press buttons to get their way.
The other mothers who have responded also sound like they have excellent advice to remedy the issue. I'm sure with the wealth of responses you have received, you will find something that works and be able to correct the problem. :)
Good luck