I have a friend. I don't really know how to respond to it, and he's been pretty flirty too. Help?

who I found out likes me, not directly from them. I saw his handwriting in our classroom, on a table that's for writing, that says (his name) + (my name). I don't really know how to respond to it, and he's been pretty flirty too. I don't want to hurt him because he's a really good friend of mine, but I don't like him in the romantic way. Help? xD