Changed his mind? Or playing it cool?

Don't worry, he probably just hasn't had the time to sit and reply. He might have just been on facebook for a few minutes but replying to your message would certainly take longer than that and perhaps he wants to think well about what he's going to say to you. I don't think he has changed his mind, maybe he just wants to take things slowly. And he does seem interested in you so I think that the best thing for you to do right now is to relax and just be patient :)