Usually when a guy has asked me what I'm doing later on that day, it typically means that he wants to do something w/you, or he wants to know if you're free so that you can join HIM in doing something.
HOWEVER...I wouldn't go TOO crazy about this though because unless you two are already in a relationship, it also proabably means that his other "choice" fell through, and you're his next best pick in line. This has sad-to-say usually been the case when guys have asked me "last minute" like this. It basically means he wants to hang with you, but he might not have thought out and planned out in advance what to do with you.
It's not a BAD thing per se, but it could just mean that his other plans w/someone else fell through and you're "next in line".
He didn't plan things in advance in other words and ask you on a formal date.
Either way, he must like you a at least a little bit to be asking you, because he must like spending time with you.
I wouldn't think too hard about it.