I just think all this over analyzing over one date and one email is ridiculous. They are just getting to know each other at this point. She emailed him. If he has any kind of guts/balls at all, he will ask her out again. In any successful relationship I've had, I have not had to bend over backwards to get the guy to ask me out again. She's expressed interest, he was lukewarm in his response. See if he has the guts and interest to pursue it. If not, is it really worth it? Like someone else said, most guys if they are interested, no matter how nervous they are, will rise to the occasion and pursue it. Why all this analyzing? I think it's stupid actually.
I go on this website occasionally, called askmen. Men talk about their dating lives and its very enlightening to get the male perspective. The first thing they would say...If he is really interested, You will know. You won't have to guess or ask or wonder or post about it. You will know, because one way or the other he will let his interest be known. And, another thing, if he was really interested, he would have called and set up a time, not given a very lukewarm, polite, response to an email.