D.L.
Sounds to me like you son is a very typical 12 yr old boy! On occassion my son still acts like that! Your son is becoming a teenager! You need to decide where you will pick your battles with him from now on.
Using your own example; would it have been so awful for him to shower at his dad's? Let me give you my own example; I believe boys should look like boys and that includes hair cuts but my son has his hair just past his shoulders. On the other hand, I don't care that his friends get PAID (allowance) for being a member of the family and doing their part (2 chores). But my son will get something (depends on cooperation) for doing EXTRA chores.
The easiest way to handle most situations is to give options when they are available. Or better yet put the situation into his lap as far as what he really wants as a consequence because of his actions/inactions. His room will be the biggest issue for you. If he has a door that can be shut, use it often!
Using your example again; "Son, please go take your shower now since you will be leaving for your dad's soon." He smarts off or ignores you. You say to him, "It's really not an issue for me if you don't but it may mess up YOUR time with your dad. On second thought, don't take a shower." That's one way and another way is; "Son, please go take your shower now since you will be leaving for your dad's soon." He smarts off or ignores you. You say to him, "I don't mind if you don't but I wonder what your dad will say when he sees you?"
I'm thinking that your house is the primary with your ex's as the secondary placement, right? If you and your ex were still married, your son would be acting this same way with him. All too soon your son will be acting awful to his dad just like with you.
Pick your battles, let things slide if you can, come up with alternatives before hand and most importantly, being able to shrug your shoulders at your son instead of raising your voice on occassion. My son and I will have our "screamfest", as we call them, but after we both are calmed down, we talk and get the situation figured out to both our likings. They don't happen as much as they used to and that's because of our talks afterwards. We try to do the talking ahead of time and skip the "screamfest" all together.
Basically, know you are not alone! I don't want to scare you but girls are much worse. My daughter and I went thru some awful, dreadful even horrible times when she was 12 yrs old til 19. Girls tend to be overly dramatic (I will just DIE if ... fill in the blanks), where as boys are plain defiant.