First off, the biggest thing to remember is that the word "parent" doesn't necessarily mean "friend." It's only natural that you'll feel the pull to be your child's friend. Now that he's older and understand more, you can start having real conversations with your tween. However, just because you can talk to each other on a different level doesn't mean that you're not still a parent. Your child still needs rules and boundaries, and you're the one who needs to guide the way and enforce those rules.
Remember what it was like to be a tween, and that may help you understand what your child is going through. Hormonal surges may leave a tween moody and sensitive, and if that's happening, try to see where he's coming from.
Figure out a way to start conversation and get him to open up to you a bit more. This may take a little practice to master, because you'll have to figure out what to say to get your child to talk. During this stage, it can take tweens some time to process their emotions and be able to discuss them. Sometimes you need to ask specific questions that will help them sort through their emotions. Other times, it may be better to let them spend some time alone so they can understand their feelings on their own.
Remember, your tween wants to be independent, but he's still looking for acceptance. While your acceptance is crucial, he also looks to his peer group for approval. Older tweens especially may feel a lot of pressure to fit in with the crowd. That's where you come in. Let them express themselves and explore who they want to be, but also keep and enforce some boundaries so that they understand their limits.