If he saw you from the back then it's very possible that he thought you were someone else. Maybe your hair color and the length is the same as someone he knows. I think he probably really thought you were someone else. I wouldn't take the shoulder tapping as an indication that he likes you in "that" way. It doesn't mean that he doesn't like you at all, it just means that he confused you with someone else. Has happened to me before too.
Don't read anything into it either way.
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No matter what your age, you don't need to change the world to find love, sometimes all that has to change is you. Be open to the possibilities.