My ex acts like he still wants a relationship. I don't. What do I do?

Long story short, we met online and were strictly long distance for a year. I went off to college and, like any 18 year old girl, wanted to enjoy my newfound freedom. I went to college partially to start over. I would've had to make what I felt to be too many unfair "compromises" if the relationship was going to last long term and, quite frankly, it wasn't going to be able to go anywhere for at least least another 4-5 years. Also, in the end, we just weren't outrageously compatible people. (For example, polar opposite political views.) I broke it off, but we remained sorta friends because I didn't wanna be an a** to him and we still got along with one another.
This was back in September.
In January, I met the goofball that I would soon fall in love with. I won't fangirl extensively here, but it's such a comfortable companionship. Awkwardness was obliterated in a blink of an eye because we just... click... On top of that, he treats me wonderfully, respects my wishes and boundaries, and doesn't pressure me into things he shouldn't. He listens. Truly listens. Something I've never really had.
I wish I could truly say that HE was my first relationship instead.
Anyway, life was wonderful until a week ago when my ex messaged me asking if I was seeing anyone. I tried to lighten the mood & casually tease him about whether he has a girlfriend yet but to no avail.
I know this is childish of me, but I'm nervous to tell him that yes, I am taken, and that I've moved on. I'm terrified of his reaction. I know he probably just wants closure, but I'm afraid of what he might or might not do when he finds out. He probably already knows the answer, but still... I really just want him to drop it and leave it alone. He knows we can't and won't be a thing again.

TL;DR Ex came out of nowhere asking if I was seeing anyone. Not sure how to react or respond.

What would you do in this situation? Am I being stupid/ignorant/etc? (Since I'm asking this at 2am, it's possible)
My ex acts like he still wants a relationship. I don't. What do I do?
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