First, tell your best friend that she had no business doing that. She may have thought she was doing you a favor, but your feelings for this guy was not her information to share. Think carefully before trusting her with additional secrets. Ask her what she said to him, and the context (e.g., what were the circumstances, did he have an audience?). There's a difference, for example, in "Jane has a crush on you,"
and "Jane has the hots for you real bad." You'll also want to know honestly whether he gave any response.
Second, he's probably either embarrassed for some reason, or he does not feel the same way. Don't treat him any differently and tell your friend you'll take it from here. When the opportunity presents itself, mention to him that you understand your friend spilled the beans about your crush on him. You could say you're sorry if it embarrassed him, then talk about it to the extent both of you feel comfortable.