Yeesh, emailing to further question why you don't want a second date? Maybe he's hoping you'll tell him what different thing it is that you want, so that he can say, "No, no....you misunderstood! I want that too!!"
Maybe you'd be doing him a favour by saying, "Look, I thought you were nice enough and although I was willing to try a second date, I still needed time to make up my mind about you. But three texts over the course of the following day when I hadn't had time to respond to any of them, only served to make me feel smothered and lose interest altogether. I am only offering you this insight so you know for the next person you date. I wish you all the best in your search."
After all, he did ask, right? So he should be prepared for whatever answer he gets. But I in no way mean to imply that you have any obligation at all to answer. He may be overly eager and clueless, or he may be a creepy stalker guy, in which case any response at all is probably a bad idea.
I do believe some of the "rules" of dating have some validity, the first being that you should never appear too eager without some kind of reciprocation of that eagerness from the other party.