How do you tell if he's just busy or not that interested?

You mostly have to take guys as they are. Perhaps there is a pattern in his contacting you? My guy was horrid at doing this--called on Friday to see what to do on Saturday. However, he also had a job and was going to the university in a tough major. Guys also spend time at home hanging out, which they need to do, and sometimes they include girlfriends. So, it isn't always what you think when you don't see much of them--they do care.