Um help ig

So theres this boy a grade below me who i call my best friend because i tell him everything and he said he is starting to have feeling fo me!!! Im the beginning i thought he was joking and i just think of him as a friend...But i dont exactly know how to tell him that without ruining our friendship.I know iy may seem like i friendzoned him HARD but what do i say to him???What do i do???Any advice?