At this point, since you already have the booking you will be significantly punished for cancelling - as others have said, however, you can if you want. First, message him, asking questions about what brings him to the area, and what he thinks his schedule might be. You might be able to frame the questions in terms of allowing you to better prepare for the visit (maybe a guidebook of he is a tourist, etc). These questions might give you a better idea of who this guy is, and with luck, might reassure you.
I always ask these questions. Nobody gets offended. Sometimes, the info is offered with the initial communication, leading me to think that most people understand that when you open your home, it is reasonable to get an idea of who this person is.
Secondly, adjust your instant book settings to require some additional verification, perhaps, and also previous positive reviews. This will prevent future similar bookings. You might also want to put into your house rules something about "drunken behavior" not being OK, which draws a clear line between something with dinner versus stumbling back to the house drunk.
Last, if you really feel unsafe either before or during the stay - pay attention and trust yourself. If you need to cancel ahead of time in order to be safe, it is worth it, and if you feel like you need to call the police during the stay, do it. Everyone's safety is paramount.
Good luck.
