I'm new here, so I want to introduce myself. Then ask if you guys have any advice.

My brother used to be a really sweet kid--really affectionate too. When I was 14 or 15, and he was around 10, my dad left , and my parents were separated for about 6 months. He didn't take this very well. Since then, he's only gotten worse. Now, he stays in his room and plays computer games all day, he never goes out with friends, and he's really irritable and angry all the time. You should see the way he talks to my mom.
Aside from this, he's always gotten straight As in school, in all honors and AP classes. But my mom just called me and told me that she got his most recent report card, which was all Cs. This completely shocked me.
Is this just a variation of normal teenage behavior (he just turned 16)? I don't remember being this bad when I was his age.
Any suggestions?