Don't worry about the meal, it's a sunk cost. I would leave that part alone and not worry about it. You don't owe someone romance because they paid for dinner and you don't have to pay someone back because you aren't interested in dating. You offered to pay, you did your part.

I wouldn't go into a lot of detail with him about being aromantic, just tell him you aren't interested in dating. Don't explain it or offer justification because it just gives him an opportunity to talk you out of it.

I'm not even sure if you have to say it today - maybe you could just wait to see if he asks again and then just decline? I would not accept any more social invites from him to avoid mixed messages.