Then say please, thank you, ask nicely the first time, etc. Model the behavior you expect him to have. Explain to him that if he does not listen the first time, then he is choosing to make you more upset, to be disrespectful, and then you are not going to be as nice.
I agree that as Mom, you have the right to pull rank as you deem fit, but he might be testing you and seeing how you are going to react. If you start out speaking to him the way you expect him to speak to you, is he going to have a come-back for that? He might also be going through a phase of being very aware of what the "rules" are and expecting everyone to follow them the same way. My DD is 3 and I can see her getting this way at some point, just because of the personality she has. Right now, when I am even just more firm in how I talk, she gets very upset and says, "Mommy not be upset! Mommy not be mad! Mommy be happy!"
I would recommend reading the book "How to Talk So Your Kids Will Listen and How to Listen So Your Kids Will Talk". It's great for learning communication strategies with your kids in a way that makes them feel heard and helps get you both on the same page.