I have an issue with a new co-worker that I'm trying to find some way to deal with it. We work in a small and very busy office and both report to different people. We're both middle aged men but while I'm quiet and keep to myself I have an active social life, he's small statured and mumbles when he talks and has never mentioned friends or outside activities. A lot of our communication in the office is by instant messenger.
After an initial period of seeming like a normal person he began coming to my cubicle over and over trying to peep on my monitor. At an employee function I was using my personal laptop during a break and he wanted to know what I was doing. I told him I was checking my personal stuff and he sat down next to me and leaned in front of me to look at my laptop. I snapped at him and later got a reply by messenger that was something like a child would write, he didn't see any problem in what he was doing.
It escalated to coming into my cubicle without warning, usually before or after another co-worker was there when no one else was around. One time I was picking up my things to go home and when I turned around he was less than a foot from me and kept wanting to talk and hang out until I had to tell him to back up. Over time he's gotten more obstinate, without any regard to how busy I am, and telling him I have things to do only has him put a strained look on his face and try marching into my cubicle again.
At a meeting recently he vocalized about trying hard to fit in and felt people were being mean to him, and yesterday it finally came to a head. We were short two people and I was very stressed trying to get the morning work done. He came over again and despite telling him I had work do to he stood there staring at me and finally became hostile, telling me I was mean to him and becoming more confrontational, denying he was snooping on me when I brought that up, and then saying he was friends with everyone when I told him I only worked with him and didn't want anything else. He left when another co-worker came in then started rapid firing me messages about everything from his social anxiety to his single relationship status to how hard he was trying, until I finally asked my supervisor to talk to him.
He met with both my supervisor and his, and what I got from the followup is he thinks people simply passing by his space and not saying anything is being "mean" and is perpetually fretting on why we're not all trying to spend time with him.
I'm at my wit's end with trying to do my job and deal with this, and don't have the time or energy to essentially babysit his social angst. What is this guy's problem and how can I get him to just leave me alone?