Confused by strange/conflicting behavior. Please help!

I recently started spending time with someone and I am constantly confused by his behavior. We've developed a pretty meaningful friendship (I care about him a lot) over the past few months, but at times he seems to shut me out without a clear reason. He can be very arrogant sometimes, whilst being openly insecure. His sense of humor is quite "raunchy" at times, both with potty and sexual humor. He has a very small circle of friends in his life, and he openly admits he has no interest in making more. Yet, he has completely opened up to me and told me about these issues of his, including his father passing away a few years ago. And then all of a sudden, he will disappear.

Since we have started spending time together, he has shown a personal interest in the things I am interested in. I am an artist, and he has shown a sudden interest in making his home very artsy; painting the walls, creating pieces of artwork, asking my thoughts on his creations, etc. He reads the authors that I read, etc.

I know that he has recently started seeing a psychiatrist to help sort out some issues with his insecurity and almost "hermit-like" behavior. He has openly admitted to me that he is incredibly nervous and insecure, and tries to shut himself out from the world because "out there" he can fall down.

It's important to note, that last week, he was acting almost "manic" with being incredibly happy and things just seemed to roll off his back.

Then, the other night, he asked me to come over to spend time with some of his friends. We were hanging out with a small group of his friends, and he and I were talking amongst ourselves. He started to say strange and pretty inappropriate things to me about situations he has been in with other women. (He had consumed a few drinks by this point, and I'm sure alcohol only aggrivated the situation.) I was getting offended for my own reasons, and I said to him, "I don't get it. Are you telling me these things to make me not like you?" The conversation hit an abrupt halt, and he said he felt really weird and uncomfortable. He made it seem like he was embarrassed and shocked by my reaction. I apologized, but it was clear things were still uncomfortable. He walked away a few minutes later, and the rest of the night he wouldn't come near me. At one point, one of his "friends" was openly making a pass at me, and being quite "grabby", which I was NOT okay with. I didn't want to make a scene, since it was his friend, but when he and I made eye contact, I gave him a "help me!" look, and he walked away. He ended up disappearing at a certain point, and I called him and walked around the neighborhood to find him; he wouldn't answer.

The next day, I didn't hear from him until the afternoon, when he sent me a text message that said : "sorry for disappearing." I said it was okay but that I wasn't okay with his friend trying to "cop a feel." He only replied with a "Yeah, just heard about that. He's a weirdo." That was 2 days ago.

I sent him an email apologizing for the remark I made, and that I really didn't mean to upset him. He hadn't responded, so I sent him a message asking if he received my email. His response was, "Yeah, thought I replied. I have no clue what you're talking about."

I don't understand. I have dealt with people with "issues" before, but I can't figure out with this guy what kind of behavior is going to come next.

I am not sure if he is bi-polar, or just depressed, or if I am just aggrivating his insecurities? Or is he just simply an inconsiderate person?

Please help.