Why do you think he is acting like so?

He is obviously angry because you don't feel the same way, he also maybe felt like you where the only one he could talk to at work, and now its the end of the world because he took your kindness for an interest in him, and he misjudged that. I would either totally ignore him back, or if you want to confront him just be blunt and say " I noticed your upset, if its something I have done I want to know?" And tell him if he's upset that you have no interest he needs to grow up and move on, tell him you don't want to ruin a friendship over something silly.