Ok, I was a foster child so I've had plenty of experience with "problem" children (was one myself!).
It could be several things causing him to act out. One, he's a teenager! Not all teenagers are alike, contrary to what some people think. Some are good kids who never give a lick of trouble, then others seem like they're possessed by demons. Then you have all the variety in between. He could just be going through a rebellious stage. A good way to counteract that, spend time with him alone, without any of the other kids, doing something he likes to do. Get your hubby to take him to a sports game. Chatting a bit during the game while he's relaxed and distracted is a good way of getting him to open up about what's going on with him. You take him out to a restaurant, a NICE one (tell him you're planning on teaching him how to date properly LOL and want him to get a bit of experience before he tries it with some poor teenage girl) and chat with him. Some teenagers just need some alone-time with a parent.
Kids who have been in the system, especially if it's for a long-term period, have trouble adjusting to living in a family-setting. Yes, he was good for the first two years, but it could have been because he forced himself to be on his best behaviour so that you'd keep him. Now that he's a bit secure, he could be acting out to test your boundaries, see how far he can push things, and all he actually needs is a bit more assurance that no matter what he does, no matter how "BAD" he is, he's still going to be YOUR kid.
There could be problems at school. Boys are always less likely to talk about stuff like that, especially boys who lived in the system because they had to be tough and defend themselves or others would think they're weak and take advantage of that and make their lives a living hell. He might just be having problems with a group of kids at school. It's not easy being adopted, especially in the pre-teen years because everyone KNOWS you're adopted. Teenagers can be the cruelest people on earth and having them know you're adopted is like telling them "Hey, my parents were druggies and alcoholics!" even if those aren't the reason the parents gave them up for adoption.
There's still a stigma to being an adopted child. Kids who've been adopted at a very young age don't usually have this problem because they usually have the same friends all through school so everyone's known about it for years and no one cares anymore about it. But a teenager who has been recently adopted doesn't have those young childhood friends to back them up.
He could just be feeling a bit neglected. I'm guessing you gave him a lot of attention when he was first adopted: helping him adapt, getting to know him, helping him in school, etc. Then, he started doing good in school and playing sports, and the attention dwindled to the same as what the other kids get. He's been used to having more attention than the others. Now he's just one of the kids and has to share equal attention and it may be hard for him.
They say that the first five years are the most important of a childs life because that's when personality develops. I'm sure that's true. But teenage years are also equally important because that's when they truly learn to be responsible adults. They also learn by example. As long as you continue to provide him with an example of what an adult SHOULD be like, he'll do just fine. It may not seem like it right now, though!
Good luck, God Bless!