It's still amazing that his body hasn't been found. I know people have mentioned suicide, jump into the river and body may never turn up, but looking at his behavior during that day and night, it did not seem he was suicidal. Why go out to a club with friends and stay until almost closing time? If it was suicide I suppose it could have been impulsive.
The most puzzling piece to this puzzle is no one sees him leave the building, which leads to that dangerous back door theory, but I agree with other posters, why leave by the scary back door to commit suicide? Why not just walk out the easier exit? This leads me to think he maybe was meeting someone out that back area or tried to because he was intoxicated? He wasn't kicked out for being overly intoxicated and appears on camera he can walk/stand without assistance.
Had he attempted suicide in the past? He had been through a lot, lots of pressure from school, job, mom passing, but it seemed like he hadn't cracked under that pressure. Did he quit the job? Did he take a break from school? Was family able to get last texts and call numbers off his phone from the cell phone company? Seemed he was close to his family, no note left behind at his apartment? No clues at all? Was he packed for his upcoming trip? I feel that there's a piece of info we're missing that may be significant.