Why does he want to know about other dates?

Red Flag.

My first suggestion is to not engage more with him at all.

But you might like him, and in this case, you say “bob, I am meeting you online, so I am assuming we are both out there so to speak. That said, I do not discuss these things with anyone I am dating as it has no place in the interaction I am creating with you. “

Why haven’t you met him in person yet?

I agree with Tallspicy that his behavior is inappropriate. You haven’t even met. Why does he think he has the right to grill you about what guys you’re dating? (The fact that you’re going out with a girlfriend is irrelevant, he is overstepping his boundaries here).

Also I would caution you against playing games in dating –“I neither confirmed, nor did I deny to keep his imagination running wild (and ofc I want him to THINK I have a date).” Just be genuine. Game playing gets you nowhere. I do think this guy is way out of line, but I also think as a general rule you should just be honest with guys and not try to project a certain image, or try to make him think things about you that aren’t true. If you’re going out with a girlfriend, say you’re meeting a girlfriend. If you have a date with another guy, why mention it at all? Don’t deliberately tell him things but leave out information to get his “imagination running wild”.