What do do when your child is rebeling against step parent??

My son is 11 years old and is lately getting into a really bad place of doing spiteful things to my partner. He will purposely not do things he is asked to do liek clean up his room or when he is toldnot to do something he will do it anyway. He does not act like this with me and when I ask him about it he just shrugs me off and wont have an answer for me. It is really becoming a soft spot in our relationship because he is feeling very disrespected and although I do not take up for my sons behavior he is growing increasingly frustrated and so am I... Any suggestions??