Without more context, I can only guess. However, there is an idiom, "to be in somebody's (back) pocket." If you are in someone's (back) pocket," it means you are totally under their control. You are completely subservient to the other person. So my guess here is that Sterling told somone* that he should place himself under the complete control of Bertie ("get in Bertie's back pocket'), and remain under his control ("and I want you to stay there"). *The paragraph is not clear whom Sterling was telling to get in Bertie's back pocket. The "you" in "I want you" could refer to Murph, or to the unnamed person telling the story (the "I" in "Murph and I").