Is his dad involved in his life? If not, what about an uncle, older cousin? This young man needs attention and example of an honorable man in his life. He needs to be encouraged to do positive things, to be feel a sense of accomplishment when he does something that's difficult and he's completed it. Seems that now he gets attention for doing the wrong stuff. We all need to be stroked and get love. He's getting it from this girl. He needs positivie reinforcement from others who love him. Tell him you need his help, have him work by your side doing things that need to be done (painting, cleaning out the garage, helping an elderly neighbor, working at a food bank or homeless shelter). When he feels a sense of value for doing something good, (that he probably didn't want to do at the outset) he will begin to feel better about himself. He'll be surprised at how postivie the experience is - even though he may moan and complain the whole time. Self-worth is a good thing but only when it's genuine based on working at something. He's 11 and there's still plenty of time to get his brain working properly.
Examine your son, serach out the things he's really good at - maybe he's not a great student, but he's excellent at putting things together (like legos) peak his interest in building, engineering, etc. (as you drive over a bridge ask him about how he thinks someone designed the bridge and how it got built? suggest he look it up online, etc.) If you attend a church see if there's a mentoring program. My almost 13 year old son can be defiant (they're right on schedule for trying to push back on the boundaries) and he's not a great student (although I know he's very bright) so I encourage him on other things that he's really good at. He's connected to our youth group and has developed friends whose families I know, he's plugged in to a mentoring group that takes a group of guys (all ages) to work on repairs of Christian summer camps each Spring and the older guys teach the younger guys. If the boys have dads the dads are invited (my husband is a policy sergeant so my son never sees his dad at work - so these weekends are wonderful). The kids help at a food pantry, they rake leaves for elderly families in our church, etc. Now that my son feels some sense of accomplishment and sees & truly understand he has value, he's a much better kid. He still gets in to trouble - but it's all small stuff (not doing homework) and he knows where the boundaries are.
There's such a disconnect between boys and their dads these days. Most boys are not great school learners. They do much better by actually doing things, moving around, copying the instructor. They're great apprentices - and that was the way things were for thousands of years until the last 100. now dads go off to work and boys are in classrooms where they are ready to pull their hair out. So they need active, positive role models.
This parenting stuff is not easy!!! Pray mama - ask for direction for your son and make sure he knows he's loved. 11 gives you plently of time to help mold the man he's going to be one day.