Is it okay to ask a guy for a second date?

sure, go for it, there's nothing wrong with that

Of course it's okay.

You could ask him out, there is nothing wrong with that. The only downside is that it could then be hard to tell the difference between him being really interested in you or trying to politely let this drop. If he is really interested he will be trying to get to that 2nd date on his own, pronto, with an invite extended back to you. In fact, at the point he canceled the 2nd date it is a little telling that he did not quickly give an alternative. I know if I am in to someone and have to cancel a date I make sure to give them an alternative date to reschedule it to show that I am interested and not just blowing them off.

The reality is that even if a date goes well, and even if a guy suggests you go out again, they can change their mind and decide that they really don't see this going much further and then try to politely stop dating you without having to directly say something to you like "I am sorry but I don't want to see you again". I mean, who wants to say that?

So, again, you could ask him out at this point and if he doesn't accept, says he is unsure, or cancels again, you need to let it drop. Or, you could give it a couple more days and see if he asks you out. If he doesn't, let it go.