* Accept graciously. If you're interested in dancing with him, simply smile and say, "Sure, I'd love to."
* Kindly decline. If you're not interested in dancing with him, you can say something like, "I'm sorry, but I'm not really feeling like dancing right now."
* Suggest something else. If you're not interested in slow dancing, but you're still open to interacting with him, you could say something like, "How about we just talk for a while?" or "I'd rather just watch the dancing for now."
No matter how you respond, be polite and respectful. Remember that the person who is asking you to dance is putting themselves out there, so it's important to be kind and considerate even if you're not interested.