Absolutely especially on his sons birthday, what possible reason could someone want to go to Bangkok alone when they are married with a kid on their sons birthday, firstly I'm surprised his wife even let him, and even more that he wanted to go.
Abandoned Mansion Bar & Restaurant on is basically opposite Soi Cowboy that is where I would start, plenty of plod down there at night and enough CCTV I'd be surprised if they didn't get a hit.