V.B.
I'm not there yet with my own kid, but boy do I remember driving my mom crazy saying that! She would just tell me that life wasn't fair and that I just needed to obey her and do what I was told. She basically let me have my rant, but it didn't change the fact that I still had to do whatever it was she was telling me to do. You may just have to ignore him when he says it or say something like "I'm sorry you feel that way, now please go get in the bathtub". It will pass when he realizes that you are going to stick to your guns no matter what he says.
I'm sorry you have to go through that! I should apologize to my mom for all the times I said it to her! :-)