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I think, and I am certainly not an expert in any sense of the word, that a smile back is nice but, you may consider speaking to him in a social gathering to see what intentions are underlying the smile. That way, you will find out what is going on and how far you want it to go and curtail anything you do not want. The guy will know where he stands as well and won't be hoping for anything more than just what you are interested in. Just my opinion. Take it with a grain of salt.
To all the comments: Luke chapter 7, verses 37-42
To all the comments: Luke chapter 7, verses 37-42