If it's the first time and he has a valid excuse, then I wouldn't think too much of it. Now that he has cancelled don't call or text him, wait for him to come to you. If he keeps trying to contact you, then he likes you. If you don't ever hear from him, consider moving on :)
1.) I didn't like her. 2.) I found somebody else. 3.) I couldn't make it. 4.) Something bad happened to the relative. 5.) I chickened out. 6.) My car got a flat tire. 7.) I ran out of money. 8.) I found peanut butter pop tarts at Walmart.