How can you tell if a guy is interested?

I recently went on a casual blind date with someone who is still in college. Our last email exchange was last Friday and he even complemented me in his email telling how cheerful I was and we were planning to meet again during Memorial Day weekend. I waited for 4 days to return his last email and then now it's been over 2 days since I sent him a reply... Well he is a first year PhD student at MIT and he is taking 4 classes and I think he may be doing research also. This week is final project due week and next week is final exam week. Shall I be more patient? If the guy is interested will he really find time to reply? Could be be busy or he is trying to think carefully before he replies? Any opinions to share? I really don't wanna be pushy and sound impatient. Thanks.

I wouldn't be too worried at this point. Students at MIT take their work very seriously because they have had to work VERY hard to get there. This time of year is very busy. try to be patient, but don't sit around waiting either.