Hi A.,
You have two issues here, one discipline and the other disrespect. Let me tell you I have an 8(girl), 10(boy) and an 11 year old (boy); when I tell them it's time to shower they all give me attitudes! But they don't push me and they don't touch me! Obviously, it's a 'shower or bath' problem :) they will cry, stomp, fuzz, moan and complain about it but they do it! They know better, and if they yell at me they get punished. I'll take the computer privileges, the tv, wii, cell phone, play station, even toys away for two days, sometimes for a week; and sometimes they've gotten a pop on their mouth for talking back or being disrespectful (such as yelling back at me).
You can't lose it girl! but you need to be firm, when they get mad at me and tell me that I am being mean for telling them to shower, I breath, smile and tell them that I could most definitely show them what "real Mean" is if they want to; then they hush.
He needs to know that you are in control and that you are the parent and he is the child. You need to be consistent with discipline, if he sees you backing down, you'll loose ground.
Usually after they get all the drama out and they are all clean, we'll sit together and watch something on tv, and spend some time together :) Or after dinner we'll bake something. Praise him for the good attitude, don't forget to do that!!!! that is very important, so he can see a balance in your relationship.
The one thing I don't agree with is to let your child chose the punishment, you can ask your child what punishment he wants, I did that once and I learned, I said "you want a spanking or privileges taken away" he answered "a spanking, cause the pain will go away quicker", so what did I do? :) you know it, I took the privileges away instead :)
So, you can't let him chose, but you sure can ask :)
Just remember, you are the mother and he is the child. Don't lose it, specially not in front of him!!! take a deep breath, turn around and walk away, think things through and then come back and let him know what the consequences to his behavior and disrespect are. And remember, you are not alone!!!!there are a lot of us out here going through it as well!!