My opinion is that there's no way he forgot about your earlier encounter, and you should be on your guard. It sounds like he may have encountered you at your workplace (?) and if so that can make it hard to tell someone you're not interested. I would simply say "I'm actually seeing someone." If he asks if it's a boyfriend or presses for more detail, simply repeat yourself that you are currently seeing someone and it's fairly new but you are not interested in meeting anyone else at present. Don't apologize. Then maybe change the subject. Good luck