What does it mean when he says, "we should do it again sometime"

No, guys very rarely BS a girl after the first date. So if he said "we should do it again sometime" it means he would like to do it again.

Sure if he had said "Well, tonight was fun. See ya!" I'd be telling you something different, but that is the line that means, quite subtly, he likes you. One sure fire way to be sure, if he was smiling and looking at you while saying it, it's true. If he was not smiling at you and looking elsewhere, there may be doubts.

No, I think the date is still on. If he's not interested, he'll say something like "well, thanks for the date. I'll see you later, take care" Or more frankly speaking, "The date was okay, but I think we don't click / are compatible."