My friend is acting strange, and I am worried

Everyone warned me against being in a relationship with a friend, but I didn't listen! Anyway, we dated for three months. Everything was absolutely perfect, he was loving, he was caring, he seemed to be serious and genuine in everything he said/did. Then we had an argument - a big argument. He disappeared for two weeks. He literally cut off all communication (something he'd never done before) and left me an emotional wreck. He finally reappeared and for a week we seemed to be back to normal. Well, not exactly - his attitude was a little different. He acted impatient with me at times, like he was annoyed.. but I chalked it up to the fight still bothering him.

At the end of the week we had another huge argument. This argument resulted in him breaking up with me (because he still cared about me as a friend but he didn't want a relationship anymore) and an abrupt end to the conversation after that. That was about a month ago. I sent him an angry letter the day after the fight and he (once again) cut off all communication. So for the past month I have heard nothing from him.

The reason we argued was because he had started changing. In the past couple of months he's made some new friends and they are a bit "out there" so to speak. They're quite rebellious and he has started to act different because of them. When we first became friends he was shy, reserved, polite, and respectful. Now he's honestly become quite mean and seems to have no regard for other people's feelings. He has pushed all of his old friends away. I tried to ignore him changing but finally I exploded on him. Now I check in on him through Facebook and he keeps deleting and reopening it.

At this point I am just devastated. He really was my best friend. I guess the only thing I can do is wait.. but I am afraid of pulling too far back because I am so worried about him. I need advice, can anyone help me? Should I try communicating anymore? It has not helped in the past.. I don't know what do.