What shall I do now? I didn't ask him what he was doing tonight when he asked me?

No one needs a relationship where you are constantly not replied to and it is just rude. If he did not have time to see your reply then he does not have time for you. You sound like a caring person and you are going to continue to wonder about him if this continues. If you can don't text him or even reply to him, see how he likes it..You don't need that in your life, good luck and I hope it works out for the best sweety!