I worked with a woman that was like that. She seemed cold, she seemed unfriendly. People made fun of her and chose to not like her. Everyday I said Hi to her, I asked how she was doing, I would sit with her at lunch and talk to her even if she didn't have much to say. I just worked with her by showing her kindness and friendship.
One day a supervisor took me a side and told me how I changed this woman's life and attitude. They said that they had never seen her smile or talk to anyone until I came a long. She had a social personality disorder and she was literally afraid to talk to people or be around them. She lacked emotion. One day she actually got the courage to start facing the public. She worked in the kitchen because that is where she felt safe. One day she told me that she wanted to work out on the sales floor with me. She was afraid at first, but then she warmed up, got the hang of it and the last time I heard, she was living a happy and better life.
Just play it cool with him. Avoid the flirting and joking stuff. Just start saying hi to him and strike conversations if it is possible you may or may not break the ice.
I hope that helps