Yes, it's kind to be cruel with some types. Be kind and straight forward first though and if he doesn't get the hint then you'll just need to stop responding to his attempts to engage you. Zero contact will seem cruel to him but it's really the nicest thing you could do for him because it will greatly help him to get over you so remember that if/when he keeps trying to have conversations with you or wants to be demoted to your platonic friend.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion